If you have very limited space for a computer user, you give him a chair, a foldaway screen, and a keyboard that includes a trackball. (There's no place to put a separate mouse.) The project I'm working on has several such keyboards. I won't name the manufacturer - I don't even know who it is. Many people call these the "PMS keyboards."
Now that title has nothing to do with the keyboards' moods. In fact, each keyboard acts like a reliable hunk, having no personality at all. But if there's no room for the mouse, where do the mouse keys go? There are three independent sets of them on this keyboard, big black keys with the white captions P, M, S.
I will not speculate about who thought up these captions and why. I'll just tell you what they mean, and why that's silly. The letters stand for:
Primary, Middle, Secondary. The default Windows setting is that "Primary" is the "left mouse" button. I think that the key names are very un-Windowish, because windows allows you to reverse the meaning of the keys if you're, say, a lefty, in which case "Primary" becomes the right mouse button. The engineers of these keyboards provided their own solution for lefties: a copy of these keys on the left side laid out in this order: S, M, P.