The division of the registers which I have made is subject to many practical exceptions, which so far I have avoided mentioning, because I wanted to fix in the reader's mind the fact that the registers are the same for all voices and are determined by the special adjustment of the vocal apparatus required for their production, and not by voice-quality. Now and then in a generation there may appear upon the scene a singer, usually tenor, who for his high notes is not obliged to adopt the somewhat artificial adjustment required by the highest register, but can sing all his tones in the easier adjustments of the lowest or middle register. But he is a phenomenon, the exception that proves the rule. Another practical exception to my rigid division of the registers is furnished by the overlapping of registers, the capacity of a singer to produce the lower notes of one register with the vocal adjustment employed for the higher notes of the register below, and vice versa; so that where the registers meet there are possibly some half a dozen optional notes. Most basses and baritones, for example, sing only in one register, that is, they carry the vocal adjustment for the lowest register into the notes they are able to sing in the register above. These exceptions will be considered later. At present, in order to treat this difficult subject in something that at least approaches an elementary manner, it is necessary to make the division of the vocal scale into registers a somewhat rigid one. It is, then, the three different adjustments of the vocal tract which determine the three divisions of the vocal scale and likewise the positions or registers for each division. The basis, therefore, for the division of voice-production into registers is not haphazard, but rests on the science of physiology. That there are not separate registers for men and women is due to the fact that men's voices run parallel to those of women at an interval of an octave below, and that, note for note, the adjustment of the male vocal tract is the same as that of the female vocal tract an octave above. For this reason basses and baritones, although singing an octave below contraltos and altos, sing in the same registers; for this reason also, tenors, although singing an octave below sopranos, employ the same registers. I am, of course, speaking of average voices, not of phenomenal ones.