The head composition is divided into two basic groups. - standard and tapered.

Standard have a lower bowl and upper bowl of the same diameter. The fork neck is also of uniform diameter. The most common dimension is 1 ⅛". On some downhill, freeride or enduro bikes, the diameter of both bowls can be 1⅛", which is distinctive for its solidity.

Tapered refers to a composition where the top bearing is the smaller 1 ⅛" diameter and the bottom bearing is the larger 1.5" diameter (see first picture).

Summary before selection:
Inspect the head tube on the frame. Are the top and bottom the same diameter or is the top diameter smaller than the bottom? Inspect the neck of the fork. Does it have a uniform diameter or does it widen at the bottom?
Here you will find top head tube compounds from German specialist Acros. Acros is part of a bearing company. This is probably where the biggest advantage of any component that has bearings and has the Acros logo comes from - they use their own bearings made for that particular product, pushing the performance noticeably further. In the case of the "headers", these are angular contact ball bearings with double seals.