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Miter, Mitre
When one piece of tubing is to be fitted tightly against another, as in the joints of a bicycle frame, the tube to be added must have its end cut in a curved shape so that it will fit against the tube it is to be attached to. This curved shape resembles the ceremonial headgear of a bishop, and takes the name "miter" from it.
A more prosaic name for this is "fish-mouth."
The accuracy and precision of the mitering of the frame tubes is fairly critical. In the case of a lugged frame, a sloppy builder can usually get away with less-than-perfect niters, since the miters are mostly not visible, hidden inside of the lugs. Even so, the frame will be stronger if the miters are well fitted. With lugged frames, the quality of the mitering can often be judged by looking at the inside of the bottom bracket, where the seat tube, down tube and seatstay miters will usually be visible.
Some investment-cast lugs are designed to provide a stepped socket that accepts a square-cut tube. This makes mitering unnecessary
