Mixed voice (voix mixte)

 

When using a correct voice technique you should not have to worry about vocal registers at all. The ideally formed voice is the mixed voice (voix mixte). This means that even when singing the highest notes, you should feel like you are using at least a small part of the chest register, and vice-versa when singing the lowest notes, you should feel like you are also using a small part of the head register.
I would like to mention the mistake of many authors, who refer to the terms middle register and mixed voice as if they were the same thing. The voice must be mixed in its entire range!!!

However, this ideal work of the vocal cords can happen only if they are given the right conditions. Some of the most important conditions, besides the correct singing posture and good work with breath, are what I call the passive larynx and a relaxed throat.

Anatomically the vocal cords are comprised of three parts. The requirement of the mixed voice is that the vocal cords must always act as one unit. This means that in the high tones, where only the edges of the vocal cords oscillate, and the other parts are "silent", the vocal cords must, as one unit, be harmonically engaged by our intonation intention. Unfortunately there is no direct way to train our vocal cords in this way. Unlike the hands of a piano player, we cannot directly watch and check them. The only way is an indirect one, which means giving the vocal chords ideal conditions for their work.

Since the movement of the vocal cords is very complex, especially when singing higher notes of each register (and the knowledge of oscillating vocal cord theory will definitely not help you sing better) I will only give you a simple description.
In the chest register, the main actively stretched part of the vocal cords oscillates.
In the middle register, the actively stretched part of the vocal cords oscillates, but not in the same width as in the chest register, but in a "narrowed" longitudinal range.
In the head register only the passively stretched edge of the vocal cords oscillates.