[larkost]

The cmd+q is the "quit" command. And the convention in single-window apps (or ones that have a single unambiguous main window) is that the window only closes when the app is quit. So this is command you have to give.

For "document-based" apps (think almost anything where you open multiple files), the application can stay running even if there are no open windows. So you have both cmd+q and cmd+w available to you.

You can probably come up with some apps that don't cleanly fit these two, but that is what Apple has.

As to screen shot commands, it is a three-key chord because it is system-wide, and they did not want to step on any toes that the apps might have. And there are a few versions: shift-command-3 takes the entire screen shift-command-4 takes either a window or a section (press space bar to switch between them) shift-command-5 opens a more menu-based system that includes a timer

Why 3, 4, and 5 (and not 1 or 2)... I don't know. Maybe there was something in those spots at some point.

[kalleboo]

Command - Shift - 1 was "Eject Floppy Disk in Drive 1" and Command - Shift - 2 was "Eject Floppy Disk in Drive 2". I kid you not, that's how old these keyboard shortcuts are, they date back to the 80's.