Following the Atari-standard, most joysticks have only one button (the "Fire 2" one). Games that support two buttons are usually optionally allowing to use the SPACE key instead instead of Fire 1.
'''Note''' Amstrad called the "Fire 3" signal "Spare", that name doesn't mean it isn't connected. On the old CPCs it was connected. However, according to the schematics, it is NOT connected on CPC+the Plus series and GX4000.
== Select Signals (COM and COM2) ==