What I love about this is back in the archaic days when car radios had (mechanical) radio buttons, that was the introductory example of a tiny state machine. They were also used in other devices but the car radio was the application everybody had seen.
Nowadays I imagine only a small proportion of people who deploy "radio buttons" have even seen such a car radio, so the metaphor is now empty, like the floppy disk icon for "save".
Random question, were physical radio buttons ever circular, or are they circular on computers only to differentiate from a check box? I've only ever seen rectangular radio buttons IRL, and a quick image search seems to agree.
I only remember rectangular ones on actual car radios, but on other apparatus they were mostly circular.
Basically a "radio button" interface is a mechanical XOR. So for example to route a signal to destination A, B, or C you'd want to push the A button and be sure B and C weren't selected. Often it mechanically latched, so you could also see at a glance which option was selected, rather than needing to have an indicator (typically a small incandescent bulb).
Radios themselves implemented that mechanism slightly differently. Each button had a stop (like a tab stop, not an organ stop). When you pushed the button it disabled all the other stops and then either a spring pulled the arm right to the stop or your own muscle power pulled it left. A pully rotated the tuner dial as the arm moved right or left. I believe the buttons were rectangular in order to have enough surface area for your finger to supply a firm press enough to pull the tuner arm all the way from right to left (the extrama case). If they'd been round they would have taken up too much vertical space.
The radios typically had five or six favorites, no more. The entire mechanism was mechanical.
Huh. I never understood the terminology "radio button", and I have no memory of noticing such a thing in a car. But it could easily have been there; why would I have cared?
I did have a tape player with buttons (play / rewind / stop / etc) that, when pushed, unpushed the other buttons. So I have a mental model for buttons of that kind. But I've never associated them with a radio.
Also, I've never associated the radio selector HTML element with a button (HTML or otherwise). Buttons are about triggering effects. But radio selectors are about selecting something; they have more in common with checkboxes.
For my tape player, the buttons were large rectangles with no border between them. If I recall correctly, they didn't sink in, but rotated - pushing one would depress one edge of the button without depressing the opposite edge.
Nowadays I imagine only a small proportion of people who deploy "radio buttons" have even seen such a car radio, so the metaphor is now empty, like the floppy disk icon for "save".