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Apple + Shift + 3 for full print screen.

Apple + Shift + 4 for a specific section of screen.
 
:apple: + Shift + 4 will let you drag an area to take a picture of, while :apple: + Shift + 4 then hitting space will change cursor from a crosshair to a camera and will take a picture of a whole window (similar to Alt+Print Screen on Windows)
 
:apple: + Shift + 4 will let you drag an area to take a picture of, while :apple: + Shift + 4 then hitting space will change cursor from a crosshair to a camera and will take a picture of a whole window (similar to Alt+Print Screen on Windows)

Not just windows, but individual cursors, the desktop, the menubar etc.
 
And if you add in 'control' to all of the above combinations, it copies to the clipboard rather than a file on the desktop (or wherever you have it save).
 
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