Found on games console joypads. One of the pair of buttons on the top edge of the pad (facing the screen). Introduced by Nintendo on the Super Famicom pad, and in that instance, otherwise known as the L & R buttons. Pushed using the index fingers, they add another combination of button pushes for things that you have to do at the same time as the functions performed by the face buttons. For example, hopping in Mario Kart, or throwing knives in Castlevania IV. (After all, you've only got two thumbs.) Something that baffled many people at first: "Bloody hell, who needs a joypad with that many buttons?" But went on to be one of those innovations that become indespensible, and are taken for granted. Almost every console joypad made sinse has featured this extra set of buttons. The PlayStation even added an extra set (L2 and R2). Now with the acceptance of analogue controls for games consoles, shoulder buttons have been phased out in favour of analogue triggers. Analogue triggers were first featured on the Sega Saturn 3-D Control Pad (aka Sega Multi Controller), and are most commonly used as accelerators and brakes in racing games.

Cheers to drinkypoo for the feedback