Similarly, some instruments are "built" in a certain key. For example, a brass instrument built in B plays a fundamental note of B, and can play notes in the harmonic series starting on B without using valves, fingerholes, or slides to alter the length of the vibrating column of air. An instrument built in a certain key often, but not always, uses music written in the same key (see trombone for an exception). However, some instruments, such as the diatonic harmonica and the harp, are in fact designed to play in only one key at a time: accidentals are difficult or impossible to play.