Control and alternate already have well-defined meanings. Control is for entering control characters, alternate is for entering alternate characters. OS X uses both. UNIX keyboards used to come with a meta key, but this fell out of use as software was written for PCs without such a key. On OS X, the usage of the command key is inherited from classic MacOS: It's the modifier that you hold for commands. This means that the OS X terminal is the only graphical terminal that I've come across that doesn't suck for copy and paste. On OS X, every single program including the terminal uses command-C for copy and command-V for paste. The terminal is therefore free to use control-C for sending the character that they terminal recognises for SIGINT. Windows overloaded the alternate key for opening menus, which meant that it is no longer a convenient key if you need to enter non-ASCII characters (for example, a Euro symbol or a letter with an accent, which are both easy to enter on a Mac). Most desktop environments for Linux inherited a load of bad UI design from Windows before adding their own mistakes.