The transport set

instructions to a tape

Unicode: U+25B6 ▶ · U+23F8 ⏸ · U+23F9 ⏹ · U+23FA ⏺ — pause, stop and record encoded only in Unicode 7.0 (2014)
The job: Control a playback mechanism — start it, halt it, hold it, capture — without words, often on a device with no screen and no room for any.
Standards: IEC 60417 registers the family for use on equipment. The shapes themselves are older than the registration and were never designed as a language: each one describes the motion of magnetic tape. The triangle points the way the tape travels and doubles for faster; the square is a full stop; the two bars are a lift-and-hold; the circle is the record head. A complete grammar of a mechanism that almost nobody using it has touched.
Failure modes: The central ambiguity is unresolved and probably unresolvable: ▶ and ⏸ share one button, and a control can show either its current state or the action it will perform. Platforms choose differently, so the same glyph means it is playing on one device and press to play on another. Separately, ⏺ record has no natural opposite, and ⏹ stop competes with ⏏ eject for the same conceptual slot.
Cultural flips: The set is a rare case where an interface deliberately does not mirror. Navigation arrows must flip under right-to-left scripts, because next follows reading order (→ VG·0002); transport controls generally do not, because they describe physical tape motion rather than reading. The result is one interface containing an arrow that mirrors and an arrow that must not, three centimetres apart.

Sources · 3

documented IEC 60417, registered graphical symbols for equipment, for the transport family.
documented The Unicode Standard: ▶ long encoded as a geometric shape, while ⏸ ⏹ ⏺ arrived only in Unicode 7.0 (2014) — decades after the buttons.
traditional The play/pause state-versus-action ambiguity as a genuine and persistent defect. Long argued in interface design; the Institute states it as a failure rather than a preference.