Serve in two capacities, as in In this company everyone is asked to double in brass occasionally. This expression was originally used in the traveling circus, where, for example, a clown was also expected to play an instrument in the street parade. The original allusion, however, is to an instrumentalist who plays more than one instrument in an ensemble, a practice particularly common among players of brass instruments. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.