An isosceles triangle pointing one vertex to the right denotes the audio or video play button.

At the beginning of the XX century, the keys of household appliances were signed with words. This is cumbersome and creates a dependency on the language. А set of universal icons was needed. There was no standardization yet, so each manufacturer drew its own characters. This continued until the mid-60s, when several major manufacturers began to label their tape recorders in a way we know today. It's hard to say who was the first. According to one version, a set of icons was invented at the Philips design bureau. On the other hand, Sony laid the foundation.

And in 1973, the German Institute for Standardization decided to use geometric shapes to indicate buttons on sound-reproducing devices. Thus, a triangle similar to the arrowhead, survived to this day as a play icon.