Teachers, and particularly male teachers, must be careful not to presume that all boys' voices change in the same manner (such as the way one's individual voice might have changed). Progressing from cambiata (first phase of change) to baritone (second phase of change) is a very individual matter and occurs in several different ways. Some boys move gradually downward by adding lower tones to the modal (also called "chest") voice and by recognizing that the head voice is becoming more and more "falsetto-ish." These boys generally experience very few "cracking" problems.
Some boys move into the second phase of change (changing voice baritones) very rapidly, and because of the fast growth, they experience considerable difficulty in controlling the voice, particularly when asked to sing above middle C. Moving above middle C requires them to move through the passaggio, the "break" or "passage" between modal and head (falsetto) phonation. Because of the fast growth of the larynx during puberty, these young men's muscularity has not developed enough to control the vocal folds when the laws of physics require the folds to adjust so they can produce tones with higher frequencies (see vocal registers). Even when they talk, they suddenly lose control and the voice "cracks" into the falsetto area.
The fast growth of the larynx has an even more dramatic effect on still others. Some boys completely lose the ability to produce any sound at all above A (top line, bass clef) other than falsetto, and it takes several months after the rapid growth period for them to begin to produce these tones. Usually the low area of these adolescent boys' voices are very strong, and they often can sing down as far as two octaves below middle C. When singing falsetto, they often experience some freedom, but there is a wide gap (passaggio) between the falsetto and the modal voice and they are completely unable to produce tones directly around middle C. As their voices settle, they usually move up several tones before maturing. Often boys who experience this type of change have had very little singing experience before entering puberty or they have been required to maintain only head voice phonation after they began the fast growth spurt. This type of voice change often occurs in young men who have been singing in a boychoir with an instructor who is reluctant to release them from singing treble. We refer to these boys as "adolescent basses."
For other boys going through the mutational process, there is a period of time in which the vocal range of the modal voice is extremely limited, from middle C down to F (fourth line, bass clef). These boys are generally referred to as being "light baritones" or "new baritones." As the days and weeks pass, they will continue to add pitches to the lower area of the voice until they become full-fledged adolescent baritones.
Due to the individual ways boys voices change and, further, due to their varied comfortable singing areas, many teachers put all the boys together (all-boys groups) during the mid-level years so they will have at least three (and four, if possible) different parts from which to choose. In four-part mixed music (boys and girls together), there are at least two boys' parts from which to choose, an option significantly better than having only one part that all boys must sing (SABoys). This variability also accounts for the reason some composers and arrangers write optional notes in the baritone part when it moves above middle C for boys with the "gap" phenomenon, or below F (fourth line, bass clef) for "light" baritones.