Good thread with great explanations. I've enjoyed reading the replies.
In my area I see a lot of 70s and 80s bike boom bikes advertised as single speed or "fixies". Most of these are butcher jobs that I feel were done simply because someone didn't know how to fix or maintain derailleurs or they think single speed and fixed gear are "a thing" but they aren't sure what that thing is. I wish those sellers could read this thread.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke