Earlier quoted context omitted.
The play button points at the direction the tape will roll in your cassette player. It had a very practical function once upon a time.
Wow, I completely forgot about that fact. So the play triangle was an arrow saying "tape is rolling in this direction" and fast-forward and reverse double triangles were "tape is rolling fast" in the respective directions. Did these buttons first show up on cassette players? The reel-to-reels I can think of had multi-position switches.
Here's an interesting reddit discussion about it with an image of an older style play button.
And an older more in depth discussion on the same thing
http://www.stackprinter.com/export?service=ux.stackexchange&...