A Margin-Worm is a soft, segmented grub as long as a finger joint, the colour of old vellum, with a dark brown head no bigger than a pinhead. Under a lamp the gut shows through the skin as a faint, shifting script, like writing seen through paper. They move in slow loops, and a book full of them gives off a faint dry ticking, the sound the Archive’s whispers make when they are being eaten. They smell of paste and old paper.

They live throughout the stacks, in bindings, between pages, and in the carpet of loose leaves on the floor, thickest where books have been merging and thinnest in the dark sections no light reaches.

They tunnel through a book, but leave no holes. What they take is the book’s unsteadiness: a page a Margin-Worm has crossed no longer rewrites itself for its reader, and a book they have eaten through is a plain book, saying one thing. They are drawn to the warmest, most restless volumes and avoid the ones that are already fixed.

They fix a text by eating its other possibilities. They cannot choose which version remains; whatever the page said when the worm crossed it, it now says for good, true or not. A worm that strays into a traveller’s journal does the same to it, and the entries stop slipping.

Moths of the same vellum colour lay eggs along the spines of the restless books, and the grubs hatch into the pages. The moths are seldom seen. The worms grow fat over a season and spin a paper cocoon in the gutter of a book.

They are harmless to the living. The Librarians are said to set them deliberately on books they want stilled, and to drive them from books they want to keep growing. Scholars who come for lost knowledge find a worm-eaten book the safest kind to read, since it will not bleed into the mind; it may also be wrong.

Visitors bring home the belief that a Margin-Worm carried out of the Archive and set on a lying chronicle will fix the truth in it. It will not: it fixes whatever the page says. A scholar’s saying, "the worms got it", is used of any history that reads too tidily.