Brass instruments, when played with no valves engaged (or, for trombones, with the slide all the way in), play a series of notes that form the overtone series based on some fundamental pitch, e.g., the B trumpet, when played with no valves engaged, can play the overtones based on B. Usually, that pitch is the note that indicates the transposition of that brass instrument. Trombones are an exception: they read at concert pitch, although tenor and bass trombones are pitched in B, alto trombone in E. Double horns are another exception, in that they combine two different sets of tubing into a single instrument, most characteristically in F and B. The horn part is nevertheless transposed uniformly in F (and indeed seldom if ever specifies whether a double or single horn is to be used), with the player deciding when to switch from one side of the instrument to the other. Single B horns also normally read from parts transposed in F.