The PC speaker is the most primitive soun system used in IBM compatible PCs. It was the only source of sound available to PC games before more technologically advanced sound cards such as AdLib and Sound Blaster were introduced as ISA plug-in cards in the late 1980s. However, even some years after these sound cards became mainstream and widely used, game manufacturers continued to support speaker sound/music in their games in order to maximise their customer base. This was in part due to the fact that sound cards did not originally follow any commonly agreed-upon standard and were largely incompatible with each other, whereas the PC speaker was the only sound system that could be regarded as universally present.