One important phase of the registers has not received much attention from the laryngoscopic investigators. This is, that most of the notes of the voice's compass can be produced at will in more than one register. Vocal teachers as a rule recognize this fact. Julius Stockhausen for instance, in his _Gesangsmethode_ (Leipzig, 1884), says: "The registers cross each other. The two principal registers of the voice have many tones in common. The perfect blending of the registers on a single tone leads to the _crescendo_, called in Italian the _messa di voce_." Teachers generally do not set hard and fast limits to the extent of each register; they direct that in singing up the scale the student pass gradually from chest to middle, middle to head voice, etc. In most practical methods the chest register occupies about the same position; this is also true of the head register. Even those teachers who profess to ignore registers recognize these two distinct qualities of tone; they instruct their pupils to sing low notes in one quality, and high notes in the other. This is in fact the general practice. In this connection the topics of registers and resonance are often combined. The terms "head voice," "head register," and "nasal resonance," are used interchangeably by the great majority of teachers. This is also true of the expressions "chest voice," "chest resonance," and "chest register." In practical instruction, the extending of the compass of the voice is usually treated, rather loosely perhaps in most cases, as a feature of the registers. Methods vary greatly in points of detail, but in most of them instruction on this topic is given along the same general lines. Usually the three classes of voices receive different treatment, one form of instruction being used for sopranos and tenors, another for mezzo-sopranos and baritones, and a third for altos and bassos. In teaching students with high voices, teachers usually "place"[5] the medium notes first, roughly speaking, from G to d (for male voices one octave lower). Then the lower notes are developed, mostly by descending scale passages, the lowest note practised being usually C. The high notes are sometimes "placed" by ascending scale passages and arpeggios, but more often by the octave jump and descending scale. There is room for considerable variation in this class of exercises, but they all conform to the same general principle. [Note 5: The expression "placing the voice" is more fully treated in Chap. VI. It is assumed, however, that the reader is familiar with the ordinary usage of this expression.] For mezzos and baritones about the same system is followed, the exercises being sung a major third or so lower. In the case of contraltos and bassos, the voice is usually trained from the middle in both directions. Most teachers favor the "chest voice" for singers of these types throughout the entire compass.