Much has been made of “the break” on clarinet—the point at which the chalumeau register and throat tones cross over to the clarion register—but all modern woodwind instruments have at least one break in their “standard” ranges. The saxophone has exactly one (ignoring the altissimo range), between the second C-sharp and the second D.
From an acoustical perspective, that point is the division between the fundamental pitches and the first overtone. When playing a lower-register note, the air column’s vibration is at its simplest. The pitch is determined by the effective length of the saxophone, which depends on which toneholes the player opens or closes. In the upper register, the air column is manipulated into vibrating twice as fast (by changing the airstream and/or opening a register vent), and a sound an octave higher is produced.
This means that there is, technically, some overlap between the registers shown above, which really are based on one specific set of “standard” fingerings. The fingerings for low B-flat, B, C, and C-sharp can be used to produce sounds in the second overtone, and the fingerings for high D through F-sharp can likewise produce sounds at the fundamental. In theory, this should mean an overlap of over a fifth:
If you’ve experimented with those fingerings, you know that they don’t work quite as expected in practice. The low B-flat fingering with the octave key added, for example doesn’t sound great, and neither does the high F-sharp fingering with no octave key. But with some experimentation, a few usable alternative fingerings can be found within this range.
