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The names of the tubes are well established for standard single-rider diamond frames, but seemingly less so for tandems.

For tandems, there's a bit more variety in frame design, but the one I've just bought - for example - has 3 lengthwise tubes plus the downtube: a sloped toptube, a horizontal tube between the 2 bottom brackets ("bottom tube"?) and a third tube in between them (I've no idea what to call that). all of these meet the head tube together.

Tandem, but what are the tubes called

The tube in the middle is common but not always used the same way - Dawes, for example, have the real toptube meet this diagonal tube behind the headtube, so the uppermost tube isn't straight in the front triangle. Others have a horizontal front toptube and very sloped rear toptube, allowing stepping through the frame.

Is there a standard set of tube names for tandems?

Chris H
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    This answer has a diagram, but does not contain any references or much in the way of discussion or alternative terminologies: https://bicycles.stackexchange.com/a/74186/452 – Will Vousden Sep 05 '22 at 09:10
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    Meanwhile, this page suggests that what's termed the "lateral tube" in the above link can also be called the "stiffener tube". – Will Vousden Sep 05 '22 at 09:44
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    @WillVousden I prefer the 2nd - "lateral" seems too much like it should mean a tube going across rather than along. OTOH "stiffener" is a good description of the purpose, less so if we want to refer a piece of metal. It establishes "keel tube" though, which I hadn't come across – Chris H Sep 05 '22 at 10:06
  • I wonder if "lateral" is more to do with the twin-tubes of a mixte, which are often thinner than a single tube. My post at https://bicycles.stackexchange.com/a/74187/19705 was a bit vague originally, and noone's managed to find a clarification. Could be the small spreader-tube is the source of "lateral" ? – Criggie Mar 09 '23 at 02:42
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    @Criggie I think I've seen a tandem with twin tubes similar to that, but I'm not sure if it had a spreader tube. That little tube certainly does run laterally. I guess a nearly-longitudinal tube could be named for providing lateral stiffening, and abbreviated – Chris H Mar 09 '23 at 06:56

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