How did medieval helmets develop from open helmets with nose guards to closed helms?

The evolution of medieval helmets from open designs to fully enclosed helms was a gradual process driven by the need for increased facial protection.12

In the 11th century, knights typically wore a mail hood (coif) paired with a pointed iron helmet featuring a nasal, a vertical bar designed to protect the nose. By the 12th century, this open design evolved into the pot helm or casque. During this transition, the helmet's crown lost its pointed shape, becoming flatter, while the nasal expanded to cover the entire face, eventually incorporating only small slits for vision and breathing.2

By approximately 1200, the helm (or heaume) emerged as a flat-topped, barrel-shaped cylinder worn over a skullcap. Because these early helms were heavy and placed significant strain on the neck, they were typically donned only immediately before combat. Over time, designs became more sophisticated, incorporating rounded contours to deflect blows, hinged visors, and the development of the basinet, which featured added neck protection and movable visors. By 1500, these advancements allowed for helmets that could be fitted snugly around the head and neck.12