What is a mahlstick?

A long stick, one end of which the painter holds in one hand, to support and steady his/her brush-holding hand. The other end of the stick is padded and rests on the panel.

Mahl sticks are relatively cheap, but they can also be handmade with a stick of wood or metal, a ball of cloth to serve as a pad, and a rubber band or string to hold it all together.