Bottom Brackets connect the crankset to the bicycle and allows the crankset to rotate freely. 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. There are various sizes of bottom bracket shell on the market, so compatibility needs to be checked. The three most common frame bottom bracket shell sizes are 68mm (English threaded road and mtb), 70mm (Italian threaded road) and 73mm (British threaded mtb). There are also many axle shapes and styles, from JIS or ISO square tapers through to the round splined axles such as ISIS or Octalink and the new external cup systems which have the axle built into the crank arms. Some newer frames are now using the new BB30 system which uses bearings pushed directly into the frame and removes the needs for cups to be threaded into the frame.