Voice leading from Wikipedia
'voice leading' is the relationship between the succesive pitches of simultaneous moving parts or voices. For example, when moving from a root position C triad or chord played C-E-G to a 6/4 chord over the same bass, you might say that the middle “voice” rises from E to F while the top “voice” rises from G to A, this being a way to “lead” those voices. Instead of considering the two successive chords vertically as separate, one focuses primarily on the “horizontal” (temporal or linear) continuity between notes, though the concept applies to homophonic as well as polyphonic musics. Concern for voice-leading often means a predominance of stepwise motion and may assist or replace diatonic functionality.
In traditional western music, voice leading is generally derived from the rules and typical patterns 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.”
'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 plays the smaller the pitch interval needed to split the sequence into two streams. Two alternating tones may produce various streaming effects including coherence (perceived as one unit), a roll (one dominates the other), or masking (one tone is no longer perceived).
Source: Wikipedia, the free encyclopedia
Voice leading - choosing chord voicings by paying attention to how the notes connect melodically to one another in a chord progression - viewing the chord progression as several melodies played simultaneously. You want to get a melody from the notes in the lead in your chord progression.
Another perspective on voice leadning is to use chord voicings to get smooth transisions from chord to chord - keeping as many individual tones as one can. This is done by changing as few tones as possible in the progression from one chord to the next. You want to keeping as many common tones as possible, and just move the notes that changes from one chord to the other, not the common tones.
So voice leading is a question of how individual tones move from chord to chord in a chord progression.
The chords in a chord progression has one or more tones in common.
Let's say you have chord change from Ami9 to D9.
|Ami9 - minor 9th|
|D9 - major 9th|
Ami9 and D9 has three notes in common, A, C, and E. So in principle, when going from Ami9 to D9 you shouldn't need to move more than two notes.
If one string plays the one note in one chord, why not let it play that note in the next chord too?
Retain fingering from one chord to the other, so as to make the movement from one chord to the other as smooth as possible musically, that is, with as little refingering of common tones as possible.
Voice leading is especially important in jazz.