It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks. The bottom bracket fits inside the bottom bracket shell , which connects the seat tube , down tube and chain stays as part of the bicycle frame. The term "bracket" refers to the tube fittings that are used to hold frame tubes together in lugged steel frames  which also form the shell that contains the spindle and bearings; the term is now used for all frames, bracketed or not. There is some disagreement as to whether the word axle or spindle should be used in particular contexts.