On typical lugged frames, the seat and down tubes and chainstays are all open to the BB shell. That's because the shell is single complex lug and there's no continuous tube inside. The basic system has open lugs so closed joints only happen at both head tube, and the seat tube/top tube joints. There's no issue at the down tube because the other end is open, but the top tube is closed at both ends so needs to be vented. most builders opt to open it to the seat tube, figuring that the post will close the hole.
Also where tubes meet tubes, the tubes are mitered to fit their neighbors within the lug to make a stronger joint.
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