Bulfinch's Mythology

The helmet was composed of two parts: the HEADPIECE, which was strengthened within by several circles of iron, and the VISOR, which, as the name implies, was a sort of grating to see through, so contrived as, by sliding in a groove, or turning on a pivot, to be raised or lowered at pleasure. Some helmets had a further improvement called a BEVER, from the Italian bevere, to drink. The VENTAYLE, or "air-passage," is another name for this.

To secure the helmet from the possibility of falling, or of being struck off, it was tied by several laces to the meshes of the hauberk; consequently, when a knight was overthrown it was necessary to undo these laces before he could be put to death; though this was sometimes effected by lifting up the skirt of the hauberk, and stabbing him in the belly. The instrument of death was a small dagger, worn on the right side.