I know that on Mac OS X, you can take a screen capture of the entire screen with Command+Shift+3 and you can take a screen capture of just a selection (or a window) with Command+Shift+4. Is there a rationale for this keyboard combination? What is the history of that command? Where does Command+Shift number originate from?
So far as I can remember/find, screenshot commands were first added to the OS itself in System 6.2
1 Text and image from FKEY that runs other FKEYs! by John Holder (MacTech magazine, 1988)
Dori's answer was correct, but I figured I'd add just a little more history. Early Mac Keyboards did not contact Function Keys (F1 through F12 etc). In place of Function Keys, they allowed the user to type Command+Shift+1 through Command+Shift+9.
Prior to Mac OS X, the system software contained a "Suitcase" called "System". This was the heart of Mac OS classic, and contained the code necessary to boot and run the machine. However this code was not located in the data fork of the file, it was located in the proprietary Mac OS Resource Fork which is set up like a database -- every resource has a type, which is a four byte value, and an ID.
Prior to Mac OS 9, all application code was in resources of type
If I recall correctly, the Command+Shift+1