There’s a market for keyboards with built-in track balls, to use where free-roving mice are a liability. Naturally the usual mouse buttons will be on the keyboard. How will they be labeled? They won’t have the symbols you have on your mouse buttons. (Take a look, they probably aren’t labeled at all; everybody knows what they are.) Maybe the keyboard mouse buttons should have little symbols suggesting Left, Middle, Right.
Well the manufacturers of the keyboard I’m using had a MUCH better idea: the buttons are labeled Primary, Middle, Secondary. But those long words don’t fit on the keys, so instead we find three buttons in a row labeled P,M, S. At first, it’s not obvious what these keys do, and the accidental acronym is ridiculous. (And by the way, to make these buttons easier on the fingers, there are TWO sets of them: on the right side of the keyboard, and ... on the right side of the keyboard.) I won’t mention the manufacturer’s name.