Where does the slang word twelves for keys come from?

The slang term "twelves" for keys likely originates from the fact that a standard piano has 88 keys, which is very close to 12 * 7 = 84.

Here's the breakdown:

* 12 is the number of notes in a chromatic scale (all the notes in an octave).

* 7 represents the number of natural notes in a diatonic scale (the major and minor scales).

So, while not a perfect match, the number 12 is significantly related to the structure of a keyboard and its notes. This proximity, combined with the common slang use of numbers to represent things, could have led to "twelves" being used as a slang term for keys.

It's worth noting that this is just a plausible theory. There's no definitive documentation on the origin of the term.