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Voice leading is the relationship between the successive pitches of simultaneously moving parts or voices. For example, when moving from a C triad in the root position (a chord played, from the lowest pitch up: C–E–G) to an inverted F chord based on the same lowest pitch (C–F–A), one might say that the middle voice rises from E to F while the highest voice rises from G to A, this being a way to "lead" those voices. Instead of considering the two successive chords separately, one focuses on the "horizontal" ("temporal" or "linear") continuity between notes in each voice. (Similar considerations apply to homophonic as well as polyphonic music.) When arranging in the Baroque, Bach-like style of harmony, the parallel movement of voices in octaves, in fifths, or in unison is to be avoided. However, popular and jazz music often contains voices moving in parallel octaves. A concern for easy voice-leading (easy, that is, for singers to read and follow) often leads to a predominance of stepwise motion and may assist or replace diatonic functionality.
In traditional contrapuntal Western music, voice leading is generally derived from the rules and patterns typical of counterpoint.
Voice leading may be described as parsimonious if it follows "the law of the shortest way" moving as few voices as few steps as possible and thus often retaining "common tones." Anti-parsimonious or circuitous voice leading is "voice leading between trichords that avoids double common-tone retention, thus requiring at least two instrumental voices to move to different pitches."
An auditory stream is a perceived melodic line, and streaming laws attempt to indicate the psychoacoustic basis of contrapuntal music. It is assumed that "several musical dimensions, such as timbre, attack and decay transients, and tempo are often not specified exactly by the composer and are controlled by the performer." An example of one law is that the faster a melodic sequence is played, the smaller the pitch interval needed to split the sequence into two streams. Two alternating tones may produce various streaming effects including coherence (perception as one unit), a roll (one dominates the other), or masking (one tone escapes perception).
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