Anyone who can yodel or whose voice has ever cracked on a high note has experienced the two parts of the voice that are called vocal registers. The lower register is called chest voice and the upper is called falsetto. Head voice is a version of the upper register that is generally thought of as a blend of falsetto and chest voice. 

The term “register” grew out of the belief that these are actually separate mechanical processes, similar to the registers of a pipe organ. I will be “pulling out all stops” to clarify what they are and how they work!  (sorry, I couldn't resist)

The registers seem to have these properties:

• The natural dividing line between them is somewhere around E above middle C for both men and women, even though women’s voices range generally about an octave higher than men’s.
• A register, in its pure form, has a short range and limited aesthetic value. This makes pure chest voice or pure falsetto somewhat useless in singing. 
• In voice training, however, the registers are very valuable.
• Most of us have some blending of the registers and a pure registration is rare.
• A pure falsetto is breathy and hooty, with less power at the bottom and is loudest at the top. Because of the breathiness, you’ll tend to run out of air and cannot sustain long phrases in falsetto.
• A pure chest voice is likewise stronger on top than the bottom of its range. It will be robust and loud, even crude-sounding.
• By blending the registers together, the useful range of your voice can be extended and each register begins to take on some of the properties of the other.

The concept of registers may not be an exact physical description, but it is a useful one for voice training. There is only one set of vocal cords so it follows that both registers are part of the same mechanism, but they do appear to represent two distinct muscular processes. For you anatomical geeks:

• Falsetto (Head Voice) is considered to be dominated by the action of the Cricothyroid muscles, which act by stretching the vocal cords.
• Chest Voice is dominated by the action of the Arytenoid group of muscles, which have several functions, namely 1) adducting the cords (bringing them together), 2) thickening the cords and 3) a specific type of adduction that’s called “damping”
• Coordinating the registers allows the balance between these vocal processes to shift as needed for efficient production of any pitch within your range, at any chosen intensity (loudness). In other words, for the lower notes in your range, chest voice will be stronger, gradually shifting to head voice dominance for the higher pitches. This creates an experience of effortlessness although it, of course, does involve muscles doing work! Good technique is not so much about “relaxation” as it is about releasing the counterproductive tensions while strengthening the dynamic reflexes and coordinated tensions that produce great singing.
• “Dynamic Registration” is a central goal of voice training. It allows for balanced coordination between the four processes of the vocal cords described as adduction, stretching, thickening and damping.

Your vocal cords operate like the lips of a trombone player, with air passing between them that causes them to vibrate, producing a pitch. There are muscles within and surrounding the vocal cords that can affect both pitch and loudness of the vocal tone, which is also similar to a trombone player's lips. As mentioned, there are 4 basic actions the vocal cords can take:

• Adduction: This is a term that means “to bring toward the centre line”. The vocal cords adduct to tightly close the airway when you swallow or hold your breath. They also adduct more lightly when you phonate (make vocal sounds). If they adduct too tightly, the sound is pinched or stops altogether. If they are too loose, the sound is breathy and weak. Fortunately, you have vocal reflexes that, with the right kind of encouragement, can find the optimal amount of adduction for speaking and singing. 
• Stretching: The cords are similar to strings on a guitar in that they can be stretched tighter, raising the pitch they produce. 
• Thickening: The vocal cords have muscle fibers embedded within them (the Thyroarytenoids, aka Vocalis muscles). When these muscles contract, they cause the cords to thicken, which lowers the pitch in a way similar to how a thicker string has a lower pitch than a thin one. The Vocalis muscle is also believed to contribute nuanced control to whether the full body of the cords vibrate or just the edges, which affects both the pitch and the richness of the vocal tone.
• Damping: In another similarity to a string instrument, the vocal cords can have the rear portion damped (adducted) together and prevented from vibrating, while the front can still vibrate. This shortens the length that vibrates, like when you move up the fretboard of a guitar.

So, with all of that complicated stuff going on inside your throat, how can you possibly coordinate it together? The answer is, you don’t have to, because the body has its own innate intelligence. What you need to do is give it a well-formed task or exercise to do and let it work out its own way of doing it. Functional Voice Training provides a reliable framework for doing this.

The case for allowing the voice to sort out its own registration issues:

Nerve impulses actually travel quite slowly - slower than the speed of sound - and processes in the brain take time to happen. This is why our bodies have reflexes and proprioceptive circuits that operate locally and don’t require centralized control to work effectively. The muscles in the larynx must respond so rapidly and precisely to the demands we place upon them that there’s no time for nerve impulses to travel back up to the brain, get processed, and return. Most of our vocal control has to happen locally within the larynx itself. By setting a stable vowel while asking the voice to sing a certain pitch, we allow the voice to adapt locally to the task we have given it…and it will do so successfully if given the chance! We just need to have a little faith in the process.