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Top Tube
From WikiPedalia
The frame tube that runs horizontally from the top of the head tube to the seat cluster. Up until the 1980's, most high quality bicycles were built with the top tube exactly horizontal. Newer frame designs commonly have sloping top tubes, higher at the front. This is particularly common in smaller frame sizes.
The length of the top tube is probably the most important dimension in providing a comfortable fit. See my article on Frame Sizing.
