When you start talking sizes rather than for a purpose, most of them are "what fraction of an em are they": https://unicode-explorer.com/articles/space-characte

When you start talking sizes rather than for a purpose, most of them are "what fraction of an em are they": https://unicode-explorer.com/articles/space-characters https://unicode-explorer.com/articles/space-characters An "en space" (half-em) is pretty close though: https://en.m.wikipedia.org/wiki/Whitespace_character#/media/File%3APunctuation-Spaces.svg https://en.m.wikipedia.org/wiki/Whitespace_character#/media/... And web browsers muddied the water a lot, since rendering two spaces is generally done by rendering two spaces, unless you're on the web and then have to do something special (some people do!   is pretty common). Personally I think this alone is the biggest influence that trained generations that one space is the majority and therefore the most correct. --- I wish they called the zero-width spaces "non-space", so we could have both   (non-breaking space) and &zwsp; (breaking non-space).