The head tube contains the headset, the interface with the fork.

The down tube connects the head tube to the bottom bracket shell.

The steering axis angle usually matches the angle of the head tube.

The short tube through which the steerer of the fork passes is called the head tube.

Not all forks steer on bearings installed in a head tube.

The frame includes the head tube that holds the front fork and allows it to pivot.

On a motorcycle, the "head tube" is normally called the steering head.

The bearing system in the head tube within which the handlebars rotate.

Riser - the metal section that raises the lip plate from the head joint tube.

The larger head tube on the Moulton gives it better hill climbing and sprinting abilities.