A register is a distinct portion of the range of the singing voice or of a musical instrument. Usually registers are defined by a change in the quality of the sound between a lower range and a higher range. Middle C, which falls almost (but not quite) at the midpoint of the piano’s 88 keys, is generally considered the dividing line between high and low.
In the case of the singing voice, the series of adjacent pitches within a given register are produced by similar vocal fold and vocal tract movements. Differences between the various registers involve many factors, including the balance between the activity of the cricothyroid muscle (CT) and thyroarytenoid muscle (TA) muscles (the TA muscle shortens the folds, whereas the CT muscle lengthens them), the balance between adduction and abduction (closing/opening) of the vocal folds, the amount of the vocal folds that is in vibration, and the shape of the vocal tract.
Chest register, or chest voice has often been used to refer to the lower range and is characterized by richness of sound. Head register, or head voice, has been used to describe the upper vocal range and is characterized by its more emotional, sob-like quality. The 'chest' and 'head' terminology reveals the historic idea of different places of resonance for different registers, which in part is still true. However, research has also revealed that the different registers are associated with different vocal fold vibration patterns.
Higher even than the head register in pitch is the whistle register.
Falsetto is sometimes described as a register but should be thought of more as style of singing than part of the range of a voice.
Musical instruments, too, have different registers. The concert flue, for example, has three recognized registers: low, medium, and high. Notes in the low register, spanning the range B3–B4, contain few overtones and can sound rather dull, dry, and hollow, which gives them a melancholy character. In the middle register, spanning C5–B5, the flute sounds mellow, bright and rich. In quiet passages the flute's middle register has a particularly graceful quality. In the upper register, C6–D7, the flute has great brilliancy and can sound penetrating and shrill.