DoubleCommand is software for Mac OS X (a kernel extension) that lets you remap keys, in other words change the way your keyboard works. Often used to make a PC keyboard more comfortable with a Mac, swapping the Alt (Option) and Windows (Command or Apple) keys, since they are in swapped positions on Mac and PC keyboards. DoubleCommand is a quick and easy way to fix this if you use a PC keyboard with your Mac. Called DoubleCommand because originally it was used to give the old PowerBooks a second Command key, where there was an enter key.

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