The manufacturer calls these "paint pens", but bird carvers often refer to them as Vermiculating pens. They hold thinned-down acrylic paints and can be used for controlled painting of fine lines, like the vermiculation on many ducks. Because you can mix whatever color of paint you want, this pen gives you superb versatility in color-matching and pigment density. Simply place a drop of diluted paint in the cup and start drawing. Works great with airbrush paints, too. These solid brass pens will last many years. Supplied with cleaning tool.