In fact, it is a symbol of rest in poetry and music (in Latin, two vertical lines are called Caesura).
In the staff, two diagonal lines indicate the pause of music; In many old English, Greek and Latin poems, two vertical lines indicate pause. Pause, pause, naturally becomes the meaning represented by double vertical lines.
As for the playing symbol, it is actually not a triangle, but an arrow "→".
Introduction of old tape recorder
When the tape is put into the tape recorder, the direction of the tape is from left to right. The arrow → indicates the direction of the adhesive tape. Later, with the popularity of electronic products, some manufacturers simplified the arrow into a triangle with a sharp corner to the right. And a small black square to stop.
In fact, the square symbol is the magnetic head on the analog tape recorder, which is the small metal square in the center of the tape cassette. Press the "play" key, the magnetic head rises, and the magnetic powder on the tape is converted into sound; Press the "Stop" key, the head will be lowered and the playback will stop.