A vocal register is a set of pitches people produce with their voices. The pitches all have the same quality, largely because the vocal cords function in the same basic way, or have the same fundamental pattern of vibration, for each note in the register, and because the pitches cause the same areas of the body to resonate.
People who have studied vocal production have separated the voice into as many as seven distinct registers, but four are accepted widely. The first of these is the vocal fry register, which is the lowest of the registers. When a person uses this register, the vocal cords are very loosely closed, and air bubbles through with a rattling quality. This register generally isn't used in singing, although it is in rare instances.
The second vocal register is the modal register, which is the vocal register people usually are in when they talk and sing comfortably. In this register, the entire vocal cord set closes. Tones in this register usually are clear and carry well, and the vocalist can produce them without strain. Much of vocal training for singing is done in the modal register.
Above the modal vocal register is the falsetto register. The falsetto differs from the modal register in that only the edges of the vocal cords, which are fairly thin, vibrate. The quality of this register is breathier, and is often described as flute-like.
The last vocal register is the whistle register, so named because of its whistle-like sound. Usually only women produce tones in this register. At least two thirds of the cords close, leaving just a small opening through which air can pass. Some singers can reach into this register quite naturally, but most of the time, some training is necessary to do so without straining and damaging the cords.
Sometimes singing instructors classify the voice into three registers instead of four: a chest register corresponding to the modal register, a middle voice and head voice. Sometimes this is divided further, with men having a chest, head and falsetto register, and women having a chest, middle and head voice. This has led to some confusion as to which register is which. "Head register," for instance, could be the middle part of the voice or the upper part of the voice, depending on gender. This is especially confusing given that the term "head register" or "head voice" is further described in some circles as the upper part of the modal register where resonance vibration occurs primarily in the head.
Regardless of whether a person divides the voice into three or four registers, the point at which a person changes from one register to another is called the break, or more properly, the passaggio. People have multiple passaggios due to the fact multiple vocal registers are present. Singers often are concerned about how to switch from one register to another, especially when they are first learning.
One key point regarding vocal registers is that the point at which people switch from one register to another is not totally constant. It is not possible for the vocal break to be the same for everyone because no two people have the exact same physiology — many different voice types exist based on the structure of the cords and surrounding tissues. Vocal teachers and pathologists have learned, however, that most people break within a fairly predictable range. For example, a soprano usually has a register break somewhere between E5 and G5.
Knowing roughly where a break occurs between vocal registers is important from several perspectives. Teachers and students use the knowledge to negotiate through the break, adjusting resonance so that there is a more seamless sound between registers and the singer appears to have a single continuous range. Similarly, composers take the typical break range into careful consideration when writing music. They know that series of pitches, particularly melismatic ones requiring a high degree of vocal agility, are harder to get through around the break points.