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In counterpoint theory, voice leading denotes the rules regulating the melodic movements of individual parts (voices), or the application of these rules by the composer. Voice leading stresses the 'horizontal', melodic aspect of a polyphonic composition, while harmony stresses its 'vertical' aspect, as the example below shows.
The score in (a) reproduces the first four measures of Bach's Preludium in C major (BWV 846a) from the Well Tempered Keyboard, volume 1.
In (b), the same measures are presented as consisting in four block chords: the first and the fourth ones are the same, a triad of C major (I); the second is a 7th chord on D (II), inverted to show C in the bass; the third is a dominant 7th on G (V), inverted to show B in the bass.
In (c), the four measure are presented as formed of five horizontal parts (voices) identified by the direction of the stems, each consisting in only three notes: from top to bottom, (1) E F — E; (2) C D — C; (3) G A G —; (4) E D — E; (5) C — B C. The four chords result from the fact that the voices do not move at the same time.
While (b) presents a harmonic reading of the passage, (c) shows its voice leading.
The rules of voice leading, in first instance, are not different from those of counterpoint and the whole question would not seem to require a separate treatment. They deal with permitted or forbidden melodic intervals in individual parts, intervals between parts, the direction of the movement of the voices with respect to each other, etc. (See Counterpoint for more details on rules, especially in Species counterpoint; see also Contrapuntal motion.)
Voice leading developed as an independent concept when Heinrich Schenker stressed its importance in "free composition", as opposed to strict counterpoint. He wrote:
Schenker indeed did not present the rules of voice leading merely as contrapuntal rules, but showed how they are inseparable of the rules of harmony and how they form one of the most essential aspects of musical composition. (See Schenkerian analysis: voice leading).
One of the main laws of voice leading in counterpoint is that the voices should make as few steps as possible and should as much as possible retain common tones between successive harmonies. In German counterpoint theory, this was known as Fließender Gesang (litt. "Fluent melody").
Schenker attributed this rule to Cherubini, but Cherubini had only said that conjunct movement should be preferred. Modern Schenkerians made the concept of "melodic fluency" an important one in their teaching of voice leading. The rule had been given by Bruckner as the "law of the shortest path", a law to which Schoenberg, who like Schenker had followed Bruckner's classes in Vienna, also referred.
Melodic fluency plays a determining role in neo-Riemannian theory, which decomposes movements from one chord to another into one or several parsimonious movements in the voice leading. The parsimonious melodic movements, however, are read here between pitch classes instead of actual pitches, neglecting octave shifts: this allows for complex voice leading, as in the example hereby.