Faithful advocates of the theory of many registers say: "Whenever in doubt about the production of a tone, sing _down_ to it from some tone above it, never _upward_ from a tone below," for they find that singing down "blends the registers." This we believe is because in singing down muscular and nerve tension is gradually relaxed and consequently there is no "register" change in the voice.
A study of the church organ will, I think, make this matter clear.

The organ has many so-called registers, as the _vox humana_, _flute_, _oboe_, etc.

These differ in the character of tone produced, because of the size and shape of the different sets of pipes and the material, wood or metal, of which they are made.

But each similarly constructed set of pipes forms only one register, and the pitch of the set varies from low to high without any abrupt change in quality.

All the tones are produced by the same methods and means, the bellows, the vibrator, and the pipe.