When it comes to the next or middle register:--[Music: F4-F5] for female voices (and an octave below for male voices), soprano and tenor sing through this entire register with ease, using a slightly different adjustment of the vocal tract from that which they employed when they went down into the lowest register. The ordinary alto stops at C in this register, as does also the bass at an octave lower. When they enter it their vocal tract adjusts itself to it and corresponds with the adjustment employed in it by soprano and tenor. In this register the vocal cords still vibrate along their entire length, but as the voice progresses upward, they show a tendency to shorten the glottic chink, and the cup, as well as the adjustment of the entire vocal tract, tends to become less open. It is the register of transition, placed between the lowest and highest, as if to bridge over the interval. The highest register: [Music: F5-C6] (an octave lower for male voice) calls for an extraordinary change in the adjustment of the vocal tract. The vocal cords are pressed tightly together at the rear and sometimes both at the rear and front. These portions thus cease to vibrate. Only the small free parts vibrate and these only at the edges. As the voice progresses up the scale the stop action ceases, the elliptical opening and the cup become smaller, and the entire vocal tract is, comparatively speaking, contracted. This register practically belongs only to sopranos and tenors. For example, although some baritones are capable of adjusting their vocal tracts to this register, their voices lose the baritone timbre, take on a feminine quality, and become male altos. In other words, there are three registers, and they correspond for all voices, but certain voices sing more in one register than in the others. Thus, the lowest register is the special province of the alto and the bass; soprano and tenor can come down only a few notes into it. The middle and the highest registers are the special province of soprano and tenor. The ordinary alto and bass can come up only part way into the middle register and cannot follow soprano and tenor at all into the highest.