Open certain files with a tool that reads their first bytes and you will find, right at the start, two letters: PK. They are not an accident and not a code. They are the initials of Phil Katz, the author of PKZIP, signed into the very front of the format he created. Every zip archive opens with his monogram, a small personal mark stamped at the head of the file like an artist's signature in the corner of a canvas.

What gives this exhibit its quiet surprise is how far those two letters reach. The zip format did not stay in the world of obvious archives. It became the hidden container for a whole shelf of everyday files that never announce themselves as zips at all. A Word document, a spreadsheet, an ebook, an Android app, a Java package, the formats known as docx, xlsx, epub, apk and jar, are all, underneath, zip archives wearing different names. And because they are zips, they all begin the same way. Half the formats on your disk secretly open with one programmer's initials, carried there by the format he wrote and never scrubbed away.

The exhibit lets you see this for yourself. Its sniffer reads a file by its first bytes, entirely on your own machine, without sending anything anywhere, and shows you the signature sitting at the front. Drop in a document you made this morning and there, before any of your words, are the letters PK. It is an oddly intimate discovery, this small trace of a single person tucked inside files you thought were nobody's in particular. The signature is the exhibit's favourite kind of object: invisible in daily use, present in enormous number, and, once noticed, impossible to unsee. Phil Katz's initials ride along inside more files than almost any name in computing, a monogram most people carry every day without ever knowing whose it is.