I've been wondering why the symbol for 'record' is universally (AFAIK - has anyone seen exceptions?) a circle and usually red. Cameras, VCRs, voice recorders, smartphones, web apps... I've seen cases where even though other controls are just labelled with text, the record button still has a little red circle beside the label.
I can make sense of most other playback controls: PLAY is an arrow pointing right, which tends to be 'forward' for LTR reading directions; PAUSE and STOP I'm a bit less sure of, but they feel like static, vertical roadblocks set up in contrast to the horizontal direction-ness of PLAY/FF/RW. My guess is that a circle represents writing to disk or something, but does anyone have actual sources about the design of playback controls?
P.S. My Google autocompletes 'why is the record button' with 'red' :V