Before we discuss the process of creating harmony in four voices, we must establish a basis as to what we mean by voice leading. Voice leading is to study how chords (and the horizontal aspect of music - melody) interact with one another. Four part writing is the traditional method of most written harmony exercises because it offers two main things: simplicity, and texture. With regards to simplicity: it is widely regarded that vocal music is easier than instrumental music. This is because vocal music does not worry about complexities of rhythm and extreme range (such as a piano). It is preferable in terms of texture because it offers the combination of the natural male voice against the female voice, which again, has to do with range. Another reason is because more complex music (such as some instrumental music) still has a basis in simply choral music. Often times it is just an expansion of the four voice practice. For this reason, throughout the study of harmony during the common practice period, we will be using four voices.
Naturally, the human voice has limitations in terms of how high and how low it can go. A more technical definition may be the breadth of pitches that a human voice can phonate. The human range of voices is generally broken into four main categories:
We discussed the process of doubling in brief in earlier chapters. As it was established, triads have only three tones. This means that, naturally, one of the tones must appear twice. Rules for doubling are based on establishing an ideal vertical sonority. While this is true, one often finds that the process of doubling is also influenced by the way voices move. Therefore, doubling is a very flexible practice. However, there are some general rules which should be regarded as absolute. The most important of these rules is that the leading tone, because of it's active tendency towards the tonic, should never be doubled when part of V or VII, or their inversions. With regards to root position triads, it is generally advisable to double the stable parts of the chord.
To achieve the best results of vertical sonority, all tones in a triad should be used. However, this is not always the case. The 5th of a major or minor root position triad can be omitted without confusing the identity of the chord itself. This is because it essentially owes it's strength to the second overtone, and the ear assumes that the 5th is present.