Headset Summary
The headset is the component on a bicycle that provides an interface between the bicycle fork and the bicycle frame itself. The short tube through which the steerer of the fork runs is called the head tube. A typical headset consists of two cups that are pressed into the top and bottom of the head tube. Inside the two cups are bearings that provide a low friction contact between the bearing cup and the steerer.

Headsets come in several different diameters, such as 1", 1 1/4", 1 1/8", and 1 1/2" (often referred to as OnePointFive), so a frame's head tube, a fork's steer tube, and the headset's diameter all have to be compatible.
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