Shield boss
A shield boss, or umbo, is a round, convex or conical piece of material at the centre of a shield. Shield bosses (or sometimes, just "bosses") are usually made of thick metal but could also be made of wood. The boss was originally designed to deflect blows from the centre of round shields, though they also provided a place to mount the shield's grip. As time went on and heater shields with curved bodies became more popular, and enarmes superseded the bar grip, the boss became more of an ornamental piece.
Often, bosses are not present on non-circular shields due to the differences in technique; with a round shield, one makes a punching motion towards an oncoming blow, while with a heater or kite shield, attacks are blocked by pivoting the shield about the body. A boss provides a significant advantage for deflecting blows when using a punching motion, but is not very effective when using a pivot to block an attack.
Manufacture
First found in prehistoric bronze, in medieval times, shield bosses were made by armourers out of sheets of iron or steel. The armourer started with a flat, relatively thin sheet and sank the metal into a bowl, which might then be planished and polished.