AppleMacintoshComputers have a ModifierKey between the ControlKey and the PropellerKey known as '''option'''.   On some Mac keyboard it is also labelled 'alt'.

In many Mac applications, the option key can be used to type some foreign characters.  For example, with a U.S. keyboard layout, option-shift-3 produces the British pound sign £. (Shift-3 normally produces #.  The converse also happens with a British keyboard layout.)  TextEdit.app is an application that recognises these option modifiers, Firefox 3 and xterm do not.