The great helm or heaume began as a cylindrical helmet reaching almost to the shoulders and gradually became tapered and developed a tapered visor. It has sights and breaths and in it's early years, may have a cross shaped reinforcement on the faceplate. It may sport a crest on it's early forms.
The great helm developed over a period of 20 years from around 1220 from the flat topped helms popular in the the late 12th century. After about 1260 the top began to develop a taper.
Between 1300 and 1340 many great helms are shown with visors (such as in the Luttrell Psalter from 1340.