I misused the term "clamp areas" I was thinking of the clamps on the bar accessories, like the shifters, when I wrote that post. I believe that's what the person asking a question was asking about.
Yes indeed, the "clamp areas" (where the stem clamps to the handlebar) on the bars themselves are different, and can vary. Most road bikes are 26.0mm and many mountain bike/flat bars are 25.4mm, but 31.8mm is common too.
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