Heater shield
A heater shield is the most recognizable shield-shape to the modern eye, with a flat (sometimes very gently curved) top and a pointed base. It was not referred to as a "heater" in period, that being a Victorian term for this style of shield, since it reminded them of a flatiron.
The heater was held at a 45 degree angle, strapped to the arm and gripped with the left hand. The upper corners of the shield protected the face and shoulder, and the pointed bottom allowed protection for the hip and upper leg. Unlike the centre-boss, the heater is not punched towards blows, but instead "rolled" back and forth using the elbow and shoulder as a pivot-point to block incoming blows.
The heater was particularly used by mounted knights in combat, but was prevalent throughout the mid- and late-medieval period.