[pavlov]

My hunch is that the square vs. circle convention is derived from paper forms.

The checkbox has been a common design element in forms for a long time. But people can of course tick off all boxes.

So when form designers needed to emphasize that you should only select one option, they often used a group of non-boxed options together with instruction copy that read “Circle one” (or similar).

The name “radio button” of course comes from physical buttons, but those were often square. So I think the specific circular shape is actually derived from circling an option on paper.

[true_religion]

I had once thought the circle shape came from scantron style examination papers, where you can only fill one circle at a time. It’s similar even if the origins are probably different.

[vel0city]

A lot of Scantron-style systems (including a lot of Scantrons) support marking multiple.

[eesmith]

Yes, I had tests in the 1980s which were 'select all that apply'.

[mystified5016]

Radio buttons were also often round. The age of radio (and phenolics) was full of over-inflated round shapes.

But also, when you have a dozen monochromatic pixels to work with, 'square' and 'round' are pretty much the only usefully distinct shapes. Checkboxes were square for obvious reasons, so to distinguish a similar set of controls, you pretty much have to use a circle.

I'm pretty sure these concepts moved directly from physical systems to digital ones. Every person alive then knew what an empty square next to a line of text meant, and everyone understood the concept of ganged push-buttons. Just map it onto a pixel grid and you're good to go

[IAmBroom]

> The name “radio button” of course comes from physical buttons, but those were often square.

It's just one opinion versus another, but in my experience early radios often had round buttons. I'm thinking of the kind of radios that preceded TVs.

Cassette decks certainly had rectangular "one choice only" buttons, but those came along decades later.