airflow of rotary valve in two positions

A rotary valve is a device used to change the pitch of a brass instrument.

Rotary valves are usually found on French horns, and sometimes on other brasses; many European classical trumpet players tend to favor rotary valves, as do some tuba players. American trumpets are almost always equipped with piston valves instead.

For much of the 19th century many professional musicians preferred rotary valve instruments as being more reliable and having faster action, but as improved designs of piston valve instruments were mass produced from the late 19th century on the rotary valve has become much less common.