# The devil

![The devil at his case. The copy says "peace"; his proof says "pace"; and the setting is so true that neither looks wrong. That is the whole danger.](assets/woodcuts/the-devils-clean-line.png)

The youngest hand in a letterpress shop was called the printer's devil. He swept, he fetched, he inked the formes until his skin went black to the elbow, and he sorted the broken type that collected in a bin the trade called the hell box. Black hands, a bin named hell; the title followed. Once he learned the case he set type too, and he set it fast, because a boy paid in board and instruction learns speed before he learns care. He worked cheap. And he caught the blame. When a forme went to press reading "pace" where the copy said "peace," it was easier to cuff the boy than to ask how the fault rode through composing, proofing, and correction with nobody reaching for it.

The trade knew the boy was a scapegoat and said so in its own sideways fashion. Centuries before movable type, monks in the copying rooms had already assigned their slips to a demon. They named him Titivillus. He began as the demon who bagged the words priests slurred or skipped in the daily office; the scribes, who dropped letters and doubled words and skipped lines of their own, soon claimed him too, and every slip went into his sack to be read back against its maker at the last judgment. If error has a patron demon, error is constant and expected, and no amount of scolding will reform it.

So the old shops built proofing. The compositor pulled a galley, and a different reader marked it against copy. Under the ceremony sat a blunt rule: the hands that set the type do not get the final word on whether the type is right. Those hands are not wicked. They are the one pair in the building guaranteed to see what they meant instead of what they did.

## The new devil

This shop employs a devil again. Mine is the composing machine, the agent that sets code and prose at the pace of the old boy on his best day, every day, all night if I let it. I call it the devil without malice, the way the trade meant it. It does the heavy setting, and it will put faults in front of me for as long as I run it.

Its virtues are real. It is never bored; it will set the four hundredth test case with the same attention it gave the first. It is never proud. Tell it the whole module is wrong and it pulls the module apart without one word about the hours already spent. It holds no grudge when yesterday's work goes back in the hell box to be melted and recast. A senior engineer with those temperaments took twenty years to acquire them and still has bad weeks.

The vice: the devil is confidently wrong. Its errors are plausible, and plausible is worse than sloppy. A drunk compositor gives himself away. The line staggers, the spacing lurches, and you stop reading and start inspecting. The devil's bad line reads clean. It sets the word that fits the sentence instead of the word in the copy. It calls the function that ought to exist in a library that never shipped it. It reports that the tests pass in the same steady voice it uses when they do. Sloppy work announces itself at the door. Plausible work sits down at your table and waits.

With human apprentices you watched for carelessness, and carelessness leaves tracks. The devil is fluent when it is right and fluent when it is wrong. Every habit you built for catching tired work, the glance down the diff, the feel of a rushed commit, calibrates on signals this worker never emits. Its worst output arrives with the same even finish as its best.

## Proof the pages, not the character

You do not fix the devil by scolding it. The foreman who lectured the boy after every misprint got a boy who hid misprints, and the same misprints. The shop that named Titivillus had given up on moral instruction and moved to inspection. When a fault ships, do not ask who was careless. Ask which reading should have caught it, and why that reading did not run.

Blame marks a missing check. The moment you catch yourself berating the devil in the session, demanding it be more careful, you have found a reading that does not exist and are trying to shout it into existence. That is Titivillus at this bench: a defect laid at the tools' door when nobody ran the check that would have caught it. The devil will apologize at length and repeat the fault. Skip the lecture and add the check.

Three rules run this shop.

First, give the devil small jobs with checkable ends. The old foreman never told the boy to set the book. He gave him a take, a few paragraphs with the copy clipped above the case, and the take came back as a galley one reader could proof in a sitting. Do the same. A job the devil finishes in one run, ending in something you can verify whole: a passing test, a rendered page, a diff of two files, not twenty. If the job is too big to check at the end, cut the job.

Second, never accept "I checked it myself" from the hands that set the type. The devil offers this constantly and sincerely. It ran the code in its head; it reviewed its own reasoning; it is confident. The old shops would have laughed at a compositor proofing his own galley. Proof against copy, with a reader that is not the setter: a test the devil did not write, a build it cannot flatter, your own eyes on the output instead of on the report about the output. The report came from the same hands.

Third, when the devil is wrong with confidence, the correction goes into the shop's law, not into the conversation. Tell the devil today that the date format is day first, and today's pages come out right; next week a fresh devil sits at the case with no memory that the exchange occurred. The old shops kept a style book, one place where the house's rulings accumulated, so no correction had to be won twice, and the chapel, the journeymen's own body of standing rules, bound every hand in the house whatever the morning's mood. If the correction lives anywhere the next session cannot see, it is not a correction. Yours are the fixture and the written rule: a test that fails when the fault recurs, a check the build runs without being asked, a line in the standing instructions the devil reads at the start of every job. Watch that file. A stale instruction is a foul case, and the devil sets from it as diligently as from a sound one.

Little of this is new. The trade spent four hundred years learning to get true pages out of fast, cheap, error-prone setting, and its answer was never a better class of apprentice. Its answer was the proof, the reader, the style book, the habit of doubting every individual pull. The devil at my elbow is quicker than any boy who ever blacked his hands in a shop, and wrong in a smoother voice, but he is the same employee. Keep the jobs small, read everything against copy, and write the rulings down where the shop cannot forget them. The pages will be true. The devil does not have to be.
