Many students of history are overwhelmed at the great diversity of arms and armour. Curators or antique dealers may need to identify the historical background of a helmet or artists and costumiers might need to accurately depict a helmet from a specific time. Medieval helmets fall into a small number of categories that can be easily generalised, though there was a great deal of regional variation.
Helms of the Early Middle Ages
From roughly the collapse of the Roman Empire until the Viking scourge had simmered across Europe, helmets were simple and uniform. Most were of the type known as a "spangenhelm," a style made of three or more plates of steel bound together into a round or conical form, somewhat like an upturned mixing bowl. For greater protection value, spangenhelms often had protective bars extending over the nose and metal or mail flaps to cover the cheeks. The Sutton Hoo helmet is an example of this. The general trend of the spangenhelm's evolution is that it became more conical, culminating in the "nasal helmet" associated with the Norman Conquest.
The Great Helm
As the knightly class became more defined, armour became more sophisticated and helmets underwent a lot of changes. During the Crusades, helmets became significantly more massive and usually covered the entire head. Shaped like a barrel with openings made for visibility and ventilation, helmets of this period are collectively known as "great helms." Helms of the 13th Century had flat tops, but through the 14th Century they evolved to become more conical.
As the great helm became more conical in the 14th century, some armour smiths took a different approach to its construction by adding hinges to the section covering the face or by omitting it entirely. The result was the basinet, which includes conical helmets that cover the top and back of the head. Some examples left the face exposed while others had face guards that could be raised and lowered. One popular variety was called the "pig-faced basinet," which had a face guard that resembled a snout or a bird's beak.
The end of the 14th Century brought drastic changes to knightly combat, and helmets became more minimalist to address the growing reliance on the longbow and gunpowder in combat. Knights began to wear helmets that had very thick domes to protect the top of the head and the back of the neck. Less emphasis was given to protecting the face and throat to allow better breathing and vision, although frequently these areas were protected by the addition of a second piece of armour (called a bevor) which was strapped around the neck.
Between the end of the Middle Ages and the abandoning of knightly plate armour, full suits of armour continued to develop among the aristocracy for both martial and ornamental usage. From the late 1400s to the 1600s they usually featured close-fitting helmets called "burgonets"--essentially re-imagined basinets that featured more articulate hinges to fit the contours of the face. Burgonets had two primary styles in that some were designed for combat while others were strictly for tournament use. Burgonets continued to be worn with a cuirass after full suits of armour were abandoned for military use, although these later examples usually lacked face protection.