8. Failure Patterns

Each pattern below is presented with its symptom, its mechanism (what actually goes wrong, as distinct from what the surface looks like), its persistence (why competent writers keep producing it), its remedy, and — indispensably — its exception: a published work that commits the fault and succeeds.

The exceptions are the intellectual heart of the section. A pedagogy of prohibitions produces writers who can avoid faults and not writers who can make choices. In every case the pattern fails because of a mechanism, not because of a surface feature, and where the mechanism is absent the surface feature is harmless.

At this scale the exceptions are unusually distinguished, which is itself an argument. Of the thirteen patterns below, the counterexamples include several novels widely ranked among the greatest in their languages. That is not an accident of selection. It is evidence that the rules describe a contemporary commercial norm with unusual precision and describe the novel as a form rather badly.

8.1 The Parallel Play

Symptom. Two chains run the length of the book without ever intersecting causally or commenting on one another. The reader experiences two books stapled together and reads one of them impatiently.

Mechanism. No convergence of either kind (§4.4). The second chain’s kernels never become the main plot’s (§3.4), and no relation of analogy or contrast is established (§3.2), so the reader has no way to convert the diversion into meaning. The failure is not that the threads do not meet; it is that nothing about either thread changes because the other exists.

Persistence. It is produced by a specific and very common drafting history: the writer had two ideas and could not choose. It also survives revision unusually well, because each thread is individually satisfying, and revision at scene level cannot see the problem.

Remedy. Two, and the cheaper one is nearly always right. Either establish a thematic relation and mark it — bring the compared scenes into proximity (§6.6), give the threads a shared object or decision-shape (§5.3), let one thread’s characters know the other’s exist. Or accept that there is no relation, and cut the weaker chain to a novella.

The exception. Tolstoy, Anna Karenina (1875–77). The Levin thread never converges with Anna’s in any plot sense, and the novel is the greater for it. The two chains are joined causally at the start — Vronsky’s pursuit of Anna releases Kitty, whom Levin marries — and thereafter touch only once, when Stiva takes Levin to Anna’s house late in Part Seven. Levin is enchanted for an evening. Nothing follows. It is a meeting without convergence, which is a structure a model requiring intersection cannot describe: the chains occupy the same room and remain causally independent.

What holds the novel together is Richard Levin’s category of analogy (the critic’s, not the character’s), sustained over eight parts and marked constantly: two people who want to live truthfully, one destroyed by the attempt and one not; two households; two forms of jealousy; two confrontations with death; two ways of failing to be understood by the person one loves. Tolstoy himself insisted, in a well-known letter defending the book’s architecture, that the connection lay not in the plot or in the characters’ relations but in an inner one, and he compared the novel’s construction to a vault whose keystone cannot be located.

This is the counterexample the chapter takes most seriously, and it is analysed at length at §9.2. It breaks the rule that a subplot must meet the main plot, and is better for it. The rule is therefore restated in this chapter as a rule about relation, not about intersection — and a student who leaves the chapter believing subplots must meet the main plot has learned the wrong thing.

8.2 The Vestigial Subplot

Symptom. A thread introduced early with apparent weight — a named character, a stated goal, several scenes — that is abandoned, or resolved in a line of reported summary a hundred pages later.

Mechanism. A breach of the promise made by placement. Per Rabinowitz’s rules of notice, material given early prominence is read as load-bearing (Chapter 9, The Opening Pages, §3.2), and a reader who has allocated attention to a thread does not silently deallocate it. The abandoned thread does not vanish from the reading experience; it persists as an unredeemed promise, and unredeemed promises accumulate as a sense that the book is not in control of itself.

Persistence. Drafting. Threads are begun to see where they go; some do not go anywhere; the opening scenes are good and survive the cut that should have removed them. Vestigial subplots are the fossil record of a book’s abandoned intentions.

Remedy. Cut backwards. The instinct is to supply the missing resolution, which costs pages the thread has already shown it does not deserve. The cheaper operation is to remove the thread’s early prominence — demote the character, delete the stated goal, convert two scenes into one — so that no promise is made.

The exception. Melville, Moby-Dick (1851). Bulkington appears at the Spouter-Inn as a figure of extraordinary promise: a tall, deep-eyed sailor around whom the narration visibly gathers. He reappears at the helm, receives a chapter of his own — a short one — and is then explicitly abandoned. Melville calls the chapter Bulkington’s stoneless grave and declines to say more.

Why does this succeed where a thousand vestigial threads fail? Because the dropping is a narrative event. Melville does not forget Bulkington; he mourns him on the page, and the refusal to develop him becomes an argument about the sea’s indifference to the men it takes. The exception yields the rule in its usable form: a thread may be dropped if the drop is marked, and cannot be dropped if it merely stops. Compare §6.4, structure 13.

8.3 The Main-Plot Hijack

Symptom. The secondary thread becomes more compelling than the primary one. Readers report skipping ahead to it; reviewers describe the book as being “really about” the subplot; the writer notices that the subplot chapters are the ones they enjoy writing.

Mechanism. The book’s centre of gravity has moved and the book’s structure has not. The reader’s investment follows interest, not outline, so the main plot’s climax arrives to a reader who has stopped caring about it — and a climax landing on an investment that has migrated elsewhere produces the flattest effect in fiction: an event of maximum significance and minimum feeling.

Persistence. Two causes, both benign in origin. Secondary threads are written with less anxiety, because less is riding on them, and unanxious writing is often better. And a subplot is usually the newer idea, so it has the writer’s current enthusiasm while the main plot has their commitment.

Remedy. Test 10 (§7.1), honestly answered. Then promote, restructure as a double plot, or — least often correct but sometimes right — deliberately reduce the subplot’s page-share and interest. What cannot be done is nothing.

The exception. Thackeray, Vanity Fair (1847–48). Amelia Sedley is the novel’s nominal sentimental heroine, and Becky Sharp is the thread that eats the book. Thackeray’s own attention visibly follows Becky; so does the reader’s; so does the critical tradition, which has spent a hundred and seventy years discussing Becky and finding Amelia trying.

The novel survives — thrives — because the hijack is converted into the book’s argument. The subtitle is A Novel without a Hero, and the narrator’s ironic management of the reader’s preferences is a sustained demonstration that we would rather watch an adventuress than a good woman, and that our preference is not to our credit. Thackeray makes the reader’s own hijack the subject. This is the discrimination the pattern requires: a hijack the book knows about is a structure; a hijack the book has not noticed is a fault. See §9.5.

8.4 The Redundant Mirror

Symptom. The subplot restates the main plot’s thematic point so closely that a reader can predict its every beat from the main plot’s, and learns nothing from the repetition.

Mechanism. Levin’s analogy collapsed into duplication (§4.1). The double plot’s whole value is that the second treatment is in a different key; where the key is the same, no comparison is possible, and juxtaposition produces emphasis rather than meaning. The reader experiences it as being told twice.

Persistence. It is the failure mode of good advice. A writer told that subplots should “echo the theme” will construct an echo, and an echo is by definition the same sound.

Remedy. Change one variable and only one. Give the second thread the same dilemma at a different social level, or the same choice with a different outcome, or the same outcome reached by an incompatible route. Contrast is easier to execute than analogy and is the safer default in a short book.

The exception. Shakespeare, King Lear (c. 1606). The Gloucester plot is the most literal mirror in the canon: a father misjudges his children, trusts the false one, casts out the true one, and is destroyed and partly redeemed. Levin and Empson both build on it, and the doubling is so exact that Samuel Johnson’s admirers and detractors have argued about it for two hundred and fifty years — A. C. Bradley judged the double action a defect in dramatic clarity while conceding that it produces the play’s impression of universality.

The reason the mirror is not redundant is that the two actions differ in every variable except their shape. Lear’s suffering is metaphysical and expressed in language of cosmic scale; Gloucester’s is physical, and its central event is an atrocity performed on stage. Lear goes mad and sees; Gloucester is blinded and sees. Where Lear’s plot asks what a king is when stripped of kingship, Gloucester’s asks what a body is when stripped of eyes. The doubling generalises the argument from a family to a world — which is the Empsonian surplus (§3.3) — and it does so by keeping the pattern and changing the register. The mirror fails when it repeats the key, not when it repeats the tune.

8.5 The Late-Arriving Thread

Symptom. A subplot introduced in the second half, or braided in twice early and then absent for two hundred pages, which converges decisively at the climax. The convergence lands on weight the book has not built.

Mechanism. Reinstatement failure (§3.7, §6.6) compounded by an investment shortfall. The reader is asked to feel a payoff proportional to an investment they were never given the opportunity to make. The felt experience is not confusion — readers reconstruct the thread easily enough — but a specific flatness at the moment of maximum intended effect.

Persistence. Late threads are frequently the solution to a plotting problem discovered in draft three, and solutions arrive at the point in the writing where the problem was noticed rather than at the point in the book where they must be planted. Writers then plant retroactively and under-plant, because the material feels obvious to them.

Remedy. Braid backwards. The rule of thumb at §6.6: a thread converging at the climax should appear in the quarter immediately preceding it, and should have been established in the first third. Two short crossings in the dormant stretch are usually sufficient and cost a few hundred words.

The exception. Tolstoy, War and Peace (1865–69). Platon Karataev enters the novel in its fourth volume, has no prior existence whatever, occupies a modest number of pages, and carries a substantial share of the book’s moral resolution: Pierre’s transformation in the prisoners’ shed is the answer the previous thousand pages have been failing to find. By every rule in this chapter he arrives far too late.

He succeeds because of a distinction the pattern must accommodate. Karataev is not a thread; he is a term. He has no chain, no goal and no reversal — he is a condition Pierre encounters, and what converges is not a plot but an idea the novel has been circling since Pierre’s first conversation about freemasonry. The reader’s investment is not in Karataev, whom they have just met; it is in Pierre’s question, which they have held for three volumes. A late arrival can pay off an early investment provided the investment was in the question rather than in the thread. That is the discrimination writers most often miss when defending a late thread, and it is a demanding one: the question must be live in the reader’s mind and must still be unanswered.

8.6 The Romance Bolted On

Symptom. A love story runs beside a plot to which it is unrelated: the detective’s relationship, the soldier’s sweetheart, the scientist’s marriage. Its beats are generic, its obstacles are misunderstandings, and its outcome could be reversed without altering a scene elsewhere.

Mechanism. Test 1 passes and test 3 fails. The thread has a chain — romance plots are easy to construct correctly, because their shape is culturally over-determined — and supplies nothing the main plot could not supply, because it is not about anything the main plot is about. The generic availability of the form is what makes it inert: a reader recognises the shape immediately and therefore attends to nothing.

Persistence. Commercial expectation in several genres; the writer’s sense that a protagonist needs a private life; and the fact that a familiar shape is quick to supply when a manuscript needs more pages.

Remedy. Make the relationship about the same thing the main plot is about, which usually means making it a site of the same conflict rather than a refuge from it. A romance whose obstacle is the main plot’s central value — the thing the protagonist will not give up — stops being bolted on and becomes the character testing ground of §4.2.

The exception. Eliot, Middlemarch (1871–72), the Fred Vincy and Mary Garth thread. It presents as the most bolted-on element in the novel: a courtship comedy, with an idle young man, a sensible young woman, and an obstacle (he must find a respectable occupation) that would be at home in a much slighter book. It occupies a substantial share of the pages and touches Dorothea’s story hardly at all.

It succeeds because it is the novel’s control case on vocation. Dorothea wants a life of consequence and is denied one; Lydgate wants a life of consequence and destroys it; Bulstrode buys one with concealed money; and Fred — the least serious person in the book — is the only character who actually acquires a usable trade, by being taught it, slowly, by a man who keeps accounts. The romance is the mechanism: Mary’s refusal is what forces the training. Reverse the courtship’s outcome and the novel’s argument about work changes. A romance is bolted on when its outcome is thematically inert, and integral when its outcome is one of the book’s answers.

8.7 The Comic-Relief Thread

Symptom. A humorous secondary line — the comic servant, the eccentric neighbour, the workplace banter — that lowers the pressure and does nothing else.

Mechanism. Pacing relief alone (§4.3), the single-domain thread that §4.8 names as the commonest justification failure. There is a second and subtler mechanism: comedy that does not know about the book’s seriousness reads as a breach of register rather than as relief, and the reader’s tonal frame is damaged rather than rested.

Persistence. The phrase “comic relief” is itself the culprit. It names the function as relief, which invites the writer to supply nothing further, and it carries the authority of long usage.

Remedy. Require the comic thread to know what the book knows. The test: could this thread exist unchanged in a book with a different subject? If yes, it is imported. The strongest comic subplots are the ones that treat the main plot’s central value irreverently — which makes them thematic counterpoint, and pays the tax twice.

The exception. Shakespeare, Henry IV, Parts 1 and 2 (c. 1597–98). The Falstaff material is the origin of the phrase “comic relief” and its permanent refutation. Empson’s reading in “Double Plots” (§3.3) is the argument in full: the tavern is not a rest from the play’s concern with honour but a sustained assault on it. Falstaff’s catechism on honour is the counter-thesis to Hotspur’s, and the two are set beside each other so that neither can be simply believed. The comic action is therefore not relief but the other half of an argument the serious action cannot make about itself.

The pattern’s proper statement follows: comic relief fails when it is only relief. Falstaff is also the exception to §8.3, since he hijacks the plays so completely that Shakespeare had to kill him offstage in Henry V rather than let him near a king.

8.8 The Occupational Subplot

Symptom. A thread that exists to give a character something to do. Usually a secondary character the writer likes, or a viewpoint character stranded between their main-plot appearances, supplied with a small pursuit to occupy the intervening chapters.

Mechanism. The thread is constructed from the outside in: the requirement was pages for this person, and a goal was reverse-engineered to fill them. Such threads fail test 2 characteristically — their turning points are satellites, because nothing depends on the outcome, and nothing depends on the outcome because the outcome was not the reason for the thread.

Persistence. Multi-viewpoint structures generate it mechanically. A book that alternates six viewpoints must give each viewpoint something to narrate, and the sixth character may have nothing to want. Chapter 13 (The Viewpoint Shift) treats the allocation problem; this is its plot-side consequence.

Remedy. Reduce the character’s viewpoint share rather than inventing business for them. A character who appears in three chapters wanting something is stronger than one who appears in nine wanting nothing much.

The exception. Tolstoy, Anna Karenina, the Oblonsky thread. Stiva has, after Part One, almost no plot: he wants a better-paid post, he sells a forest badly, he arranges dinners, he owes money. It is a set of small pursuits invented to keep a character in motion, and it occupies a real share of a very long novel.

It works for two reasons that convert an occupational thread into a functional one. First, Stiva is the novel’s crossing character (§5.3): he is Anna’s brother, Dolly’s husband, Kitty’s brother-in-law and Levin’s friend, and almost every crossing between the two great chains passes through him. His small business is the connective tissue the novel’s structure requires. Second, his small business is the novel’s economic argument: the forest sold below value to a merchant, the sinecure sought through connections, the debts that Dolly’s household absorbs. Levin’s agricultural chapters state the case about land and work directly; Stiva’s dinners embody its opposite. An occupational thread is redeemed when the occupation is the point.

8.9 Convergence by Coincidence

Symptom. The threads join because of an improbability: the two characters turn out to be related, or to have been at the same place years earlier, or to arrive at the same door on the same night.

Mechanism. A violation of the causal standard the book has taught the reader to apply. Chapter 2 (Scene and Sequel) establishes that a story is a chain of things that had to happen next; a coincidence is by definition a thing that did not have to happen, and placing one at the load-bearing joint is placing the weakest available material where the most weight falls. The reader’s objection is rarely to the improbability itself — real life is full of them — but to the convenience: the coincidence serves the author, and its serving the author is visible.

Persistence. Convergence is hard and coincidence is easy. It also frequently enters late, as the repair for a structural problem discovered in revision.

Remedy. Two, in order of preference. Move the coincidence earlier: an improbability at the beginning is a premise and costs nothing, while the same improbability at the climax is a cheat. And make the coincidence caused by character: if the two threads meet because both people wanted something that made them go to that place, the meeting is motivated even if its timing is lucky.

The exception. Dickens, Bleak House (1852–53). The novel runs on coincidence at a density no contemporary editor would permit. Esther is the illegitimate daughter of the woman whose portrait she happens to resemble; the law-writer whose hand Lady Dedlock recognises is Esther’s father; Jo carries smallpox from the graveyard to the girl who is the dead man’s daughter; Guppy, Krook, Nemo, Miss Flite and the Jellybys are all connected.

The coincidences are not defects excused by genius. They are the novel’s thesis in structural form: that a society which believes itself divided into separate compartments — Chesney Wold and Tom-all-Alone’s, Chancery and the brickmakers’ cottages — is in fact one connected system in which infection, guilt and money travel freely. Dickens states the argument explicitly when he asks what connexion there can be between the place in Lincolnshire and the outcast sweeper, and the plot’s improbabilities are the answer. Coincidence is a fault when it is a convenience and a technique when it is a claim about the world.

The lesser exception is instructive for the opposite reason. Charlotte Brontë’s Jane Eyre has Jane collapse, starving and anonymous, at the door of her own cousins — one of the most notorious coincidences in English fiction, and one Brontë neither disguised nor justified. It survives because the novel’s mode is providential and the coincidence is legible as providence rather than as convenience. The genre licenses it. A realist novel of the same period could not have done it.

8.10 Convergence by Encounter

Symptom. The threads converge because the characters meet. They are in the same room at the end; they are introduced; they speak. Nothing else changes.

Mechanism. A meeting is not a convergence. Applying test 5 (§7.1), the meeting does not break the main plot’s logic if removed — one could delete the encounter and both chains would proceed unaltered. This is the commonest false positive in the whole protocol, because the meeting looks like the thing convergence is supposed to be, and writers mark it on their outlines as the point where the threads join.

Persistence. The visual convention of screen narrative, where the convergence of storylines is very often literalised as an encounter and where the actors’ shared frame does real work. Prose has no shared frame; a meeting must produce a consequence or it has produced an image.

Remedy. Ask what each character wants from the other, and make the wants incompatible. Convergence is conflict, not contact. If neither party wants anything from the other, the encounter is a curtain call and belongs at the end, if anywhere.

The exception. Joyce, Ulysses (1922). The whole book runs towards the meeting of Bloom and Stephen, and when the meeting arrives it is deliberately, almost aggressively inconsequential: two tired men, cocoa, a conversation at cross purposes, an offer of a bed declined, urination in a garden, and a parting. Nothing follows. Stephen does not become Bloom’s son and Bloom does not become Stephen’s father, except in the sense that the reader supplies.

This is not a failed convergence but an anti-convergence, and it works because the novel has spent seven hundred pages establishing that contact between human beings is this partial. The two chains have been converging thematically all day — the Odyssey structure, the paternity motif, the parallel exclusions — so that the encounter’s emptiness is felt against a convergence that has already happened elsewhere. An encounter can be the convergence when the emptiness of the encounter is the point, and only then.

8.11 The Digression

Symptom. A long passage of non-narrative material — history, technical exposition, essay, catalogue — occupying the page-share of a subplot. No chain, no goal, no reversal.

Mechanism. It fails §2.1 outright and every test in §7 by definition, since the tests assume a chain. The reader’s experience is of the book stopping, and Chapter 7 (Backstory and Flashback) describes the resulting interest deficit: an open question is suspended and nothing narrative is offered in exchange.

Persistence. Research; conviction; the writer’s belief that the reader needs this to understand. All three are frequently correct about the need and wrong about the delivery.

Remedy. The standard one is to distribute the material through scenes. It is the right advice for most manuscripts and it is worth noticing what it costs: distributed exposition cannot make an argument, because an argument requires continuity. A writer who actually has an argument to make may reasonably decline the remedy.

The exception, or rather the two exceptions, which are among the largest in the tradition.

Hugo, Les Misérables (1862). The Waterloo digression occupies a substantial book of the novel and describes a battle at which none of the principals is present. The essay on the Paris sewers, the history of the convent, the disquisition on argot — all fail every test in this chapter. And Hugo does something with Waterloo that repays close study: at the very end of the digression, in its last pages, Thénardier robs the dead on the battlefield and in doing so appears to save the life of Marius’s father, which creates the obligation that governs Marius’s entire relation to the Thénardiers. The digression terminates in a kernel. A hundred pages of history are converted, in a paragraph, into the load-bearing joint of a subplot. Whether that redeems the hundred pages is a legitimate critical disagreement and is set as a seminar question at §12.2. The sewer digression works differently: it precedes Valjean’s carrying of Marius through those sewers, so that the reader traverses the described space with him. It is a descriptive pause functioning as preparation, and it is the clearest case in the tradition of a digression paid for by what follows it.

Melville, Moby-Dick (1851). The cetology chapters are more radical, because they are not preparation for anything. They are an attempt to know a whale by classification, measurement, drawing, history and dissection, conducted at length, and their function is to fail: the accumulation of knowledge that does not produce understanding is the novel’s argument, and the argument cannot be made in three pages because its subject is exhaustiveness. A digression can be justified by being unbearable, which is a defence no craft protocol can accommodate and which a chapter that suppressed it would be dishonest.

See §9.6 for the full treatment, and §16.

8.12 The Metronome Braid

Symptom. The threads alternate on a fixed schedule — every chapter, or every other chapter — regardless of what either thread needs. Crossings occur where the pattern says rather than where the material says.

Mechanism. The braid has become a container rather than a rhythm. Two costs follow. Threads are cut away from at the wrong moments: a scene reaching its disaster is interrupted because the rotation demands it, and a thread with nothing to report is visited because its turn has come. And the reader learns the pattern, which converts every crossing into an expected event and eliminates the emphasis a departure from pattern could have carried (§6.4, structure 1).

Persistence. Regular alternation is easy to plan, easy to hold in the head during a long draft, and reassuring. It is also the structure most likely to survive from an outline into a finished book unexamined.

Remedy. Vary the interval by need, and reserve regularity for stretches where it can be broken to effect. A practical procedure: mark on the chapter chart (§6.5) every crossing that occurred because the material demanded it; if fewer than half are marked, the braid is a metronome.

The exception. Steinbeck, The Grapes of Wrath (1939). The intercalary chapters alternate with the Joad narrative on something very close to a fixed schedule, and one strand of the braid is not a chain at all — the general chapters have no continuing characters, no goal and no reversal. By this chapter’s definitions the structure should be inert twice over.

It succeeds because the regularity is the form’s argument. The alternation asserts that the Joads’ particular journey and the general migration are the same event at two magnitudes, and a regular alternation makes the assertion continuously without ever stating it. Vary the interval and the claim weakens. Chapter 2 (Scene and Sequel) reads the intercalary chapters as collective sequel; the structural point here is complementary — a metronome braid is a fault when the regularity means nothing and a technique when the regularity is the claim.

8.13 The Thematic Alibi

This is the failure the domain model itself generates.

Symptom. A thread defended, in workshop or in the writer’s own head, on the ground that it “explores the same themes”. No other justification is offered, and none is available.

Mechanism. The Empsonian defence deployed without the Empsonian mechanism (§3.3). Any two narratives placed side by side can be described as commenting on each other; the description is unfalsifiable and therefore worthless as a justification. The alibi is dangerous because it is true — the thread does share the book’s themes — while being irrelevant, since sharing a theme is a property of almost everything a writer writes.

Persistence. It is the only defence available for a thread the writer cannot bear to cut, and it is socially unanswerable in a workshop, where disputing it looks like a failure of subtlety.

Remedy. Test 4 (§7.1), applied strictly. Require the writer to complete the sentence because this thread exists, the reader understands that the main plot’s situation is qualified in the following way… and to name the two scenes between which the understanding is produced. A thread that cannot survive this has an alibi rather than a function.

The exception. Tolstoy, Anna Karenina, the Koznyshev and Varenka episode. In Part Six, Levin’s half-brother Sergey Koznyshev goes mushroom-picking with Varenka intending to propose. Both know it. He begins to speak, cannot, and says something about the difference between two kinds of mushroom; she answers; the moment passes, and neither ever refers to it again. The episode occupies two short chapters, has no consequence for any chain in the novel, and exists — by any structural account — to rhyme.

Its rhyme is with the novel’s other proposal scenes: Levin’s first, humiliating refusal; Levin’s second proposal, conducted in chalk initials, in which two people understand each other perfectly without completing a word; Vronsky’s approach to Anna, which is not a proposal and cannot be. Set beside these, Koznyshev’s failure asserts something the other scenes cannot: that the capacity to say the thing is not distributed according to merit, intelligence or even desire, and that a life can be decided by two minutes in a wood. It is thematic counterpoint doing everything and nothing else, and it is among the finest passages Tolstoy wrote.

The discrimination. The alibi fails when the thread’s thematic relation is asserted; it succeeds when the relation is executed — when the reader, without being told, holds two scenes together and something occurs. The difference is not in the defence but in whether the book has done the work of proximity, marking and contrast that makes the comparison happen. §6.6 is that work.

8.14 Summary: symptom to mechanism