From the Nintendo era onward, the vast majority of video game controllers have named their face buttons after letters in the alphabet — almost always picking from A, B, C, X, Y and Z — with the exception of the Sony Playstation and its progeny.
I never thought to question the Playstation’s combination of square, circle, triangle and X, but the folks at Famitsu magazine did. 1UP relays the magazine’s conversation with Sony designer Teiyu Goto:
We wanted something simple to remember, which is why we went with icons or symbols … I gave each symbol a meaning and a color. The triangle refers to viewpoint; I had it represent one’s head or direction and made it green. Square refers to a piece of paper; I had it represent menus or documents and made it pink. The circle and X represent ‘yes’ or ‘no’ decision-making and I made them red and blue respectively.