When people talk about Chicago’s vernacular architecture they inevitably talk about 3 flats and courtyard apartments. But honestly, you can find pretty similar forms in nearly every city in the US. For a truly unique Chicago form, you need to look to the Four Plus One.
Everybody loves to hate on the humble Four Plus One. I think it’s mostly received wisdom from the relatively well off single family home residents (read NIMBYs) that would have rather left housing density to be somebody else’s problem.
Personally, I think they’re great. They are the Mid Century Modern of housing. If you’d like to know more, Forgotten Chicago has a great write up of the style.