The bottom bracket of a bicycle is an ensemble that contains a spindle to which the cranks are attached and bearings which allow the spindle to rotate. It fits inside the bottom bracket shell, which is found at the bottom-centre of the bike frame, usually at the bottom of the seat tube.

There are two main types of bottom bracket axle, splined and square. Square taper is an old standard which has been in use for decades on all bikes. Recently there has been a move to using splined cranks on mountain bikes as it is technically stiffer and should last longer. The cranks you are using will determine which type of bottom bracket is needed. The two main types of splined bottom bracket are ISIS (International Splined Interface Standard) and Octalink. Octalink is only used on some Shimano cranks, and as their cranks aren't best suited to trials this type of axle is rarely encountered in trials. The two standards are NOT compatible.

There are two widths of bottom bracket shell commonly found on trials bikes, 68mm and 73mm. You can measure this width easily using a ruler or tape measure - just measure from one edge to the other. Leeson frames use a different size bottom bracket and require a special bottom bracket installed at the time of creation.