A key is just a physical copy of a short number

A dark card, headed BITTING. A small ruled table of numbers, one row per pin chamber. The silhouette of a key beneath it, and a line at the foot: DO NOT KEEP WITH THE KEY.

Read the numbers correctly and you don't need the key at all.

What is a bitting code?

A list of depths, one per pin chamber, in the order they sit along the key.

Every pin tumbler lock sorts on depth — each chamber cut to a specific level — and the code is nothing more than those levels written down as digits. A locksmith reads the string the way a machinist reads a tolerance: not as a description of the key, but as an instruction for cutting one.

Why does the number do the same job as the metal?

Because the metal is only ever a rendering of the number.

Cut a blank to the recorded depths on a code-cutting machine and the result opens the lock exactly as well as the original does, whether or not the machine — or the person running it — has ever seen the real key. Nothing about the brass matters except where the cuts sit. The key is a delivery mechanism for the code, and the delivery mechanism can be skipped entirely.

That's the whole reason the card is a different object from the key, with different rules attached to it.

Which lock is worth putting on a shirt?

One with a code series that means something to the wearer, not the highest security rating in the shop.

A locksmith's own bench work, a rekey they're proud of, the master system for a building they maintain — the specificity is what makes it read as real rather than as a diagram of the idea of a key. It's the same logic that makes a wiring diagram read as insider knowledge rather than decoration: the shirt has to be able to be marked right or wrong, the same way a cobbler's last is unmistakably one particular foot and not a shoe in general.

Can this show a real manufacturer's code series?

No, and this is the one hard boundary in the piece.

A manufacturer's code series identifies that maker's product line — reproduce it and you've copied part of a proprietary system, not just drawn a picture of a number. A trade accreditation mark carries the same problem from a different direction: it asserts a body vetted the wearer, and nobody licenses that claim to a T-shirt. Invented digits with no maker's name sidestep both problems at once, and to anyone who isn't a locksmith — which is everyone who'll see the shirt — an invented series reads exactly as real as a genuine one.

Does this work for a whole ring of keys, not one?

It works better for one.

A service card can hold five entries and gain meaning with each one; a bitting card can't — the moment it holds more than one code it stops reading as a specific lock and starts reading as a spec sheet. If there's a second code worth printing, that's a second shirt, not a busier version of this one.

Doesn't printing a code just teach someone to pick a lock?

No — bitting has nothing to do with picking.

Picking is a way of working around not having the code. Cutting a key from a bitting code is the opposite situation: you already have the number, which almost nobody legitimately does, and the interesting fact isn't that the number opens the lock — that's the whole point of a key — it's that the number is completely inert without a lock to hold it against, and a shirt is not a lock.

Does a bitting card print well?

Yes — it's a ruled table and a silhouette, close to the easiest artwork there is.

The numerals want to stay large: the original is written for someone standing at a bench, and dropped to shirt scale the fine hairline rules can go before the digits do. Charcoal with sepia or cream keeps it looking like something pulled from a drawer rather than designed to be worn, and that's exactly the register flat colour and hard edges are built for — one ink beyond the base colour is enough, with a second colour reserved for the key silhouette.

How do I get one made?

Describe the code at JustOG — real or invented, how many chambers, and whether the key silhouette sits above or below the table. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped.

Designs other people have published are in the shop.

A card left on a bench does nothing. A card left with the key does.