The round brush is the most common watercolor brush, characterized by a round "tuft," or bundle of brush hairs. Aside from that, a round brush can have countless other variations, like the size of the tip, the flexibility of its hairs and whether the tip is convex or pointed. Extra fine or large round brushes have their own special classifications, such as "Rigger" or "Mop."
The tuft of this brush is laid out flat, although it may end in a straight, angled or rounded edge and in a variety of lengths. Short tufts tend to be very stiff, ideal for square corners, straight edges and even lines, while long ones are more flexible and good for covering large areas with even color. The brushes are measured by the width of the tip, and the size can be found imprinted on the ferrule (the metal tube which holds the tuft to the handle).
Oval wash brushes, or "Mops," are a specialty type of round brush used to apply a wash to a painting. Wash is the technical term for filling in a large area of the paper with a single color, which then becomes the background of the painting or the base of a graded or layered effect. Oval wash brushes are characterized by a wide, convex tip on the tuft, perfect for holding an ample amount of paint and applying it to paper evenly.
The rigger is a specialty round brush made for rendering fine lines. The tuft is made of long (sometimes very long) hairs that taper down into either a pointed, flat or angled tip. The rigger got its name from its ability to paint long lines of even width needed for a ship's rigging. The brush is also used for tree branches, blades of grass and the upper part of a bridge.
A hake brush is a special type of flat wash brush with an extra-wide width. It's used to cover large areas with a single color wash or for more expressive strokes. Most hake brush tufts are made of goat hair which is noticeably more coarse than sable or nylon hairs, but also more flexible.