Purpose of four-part writing
The best way to study harmony is a four-part (four-voice) texture. By the texture, we mean the way how melodic, harmonic and rhythmic materials are combined in a composition. Four-part writing allows us to see both the harmonic relations in chords and melodic relations in each individual voice, to write a smooth melodic connections from note to note, generating chords. When we write independent melodies simultaneously, put one pitch against the other pitch (punctus contra punctum) there will be several types of contrapuntal motion: similar, parallel, contrary and oblique. Similar motion - melodies (voices) move in the same direction. Parallel motion - voices move in the same direction keeping the same interval. Contrary motion - voices move in opposite directions. Oblique - one voice stays at the same position and the other moves.
Voices in four-part texture are bass - the lowest voice, tenor, alto and soprano - the highest voice. The range of each voice is an octave and a half. Chords can be in two positions: close and open. In close position, upper voices lie thirds or fourths apart. In open position, upper voices lie fifths and sixths apart. To build four-part harmony, you need to double some tone of a triad. Usually, composers double the root.
The connection of different chords can be harmonic when the common tone of two chords stays in the same position; melodic when any tone does not stay at the same position. When roots of the chords lie fourth or fifth apart connection of the chords can be harmonic or melodic. Moving upper voices go in the same direction. When roots lie third apart connection can be harmonic and melodic. When roots lie second apart connection can be only melodic because the chords have no any common tones. Three upper voices go in the opposite direction to the bass.
Basic rules of four-part writing
There are also some basic rules of four-part writing that we must remember. Avoid parallel perfect consonances: octaves, unisons, fifth. Avoid augmented melodic intervals in all voices. The leading tone usually goes upward to the tonic. The unresolved leading tone sounds painful. Do not double the leading tone of the scale. Try to avoid crossing voices. Try to keep the distance between neighboring upper voices no more than an octave. However, the distance between bass nad tenor can be any acceptable interval.