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Voice leading is a term referring to how individual voices move within a melodic line. Changing notes leads to different harmonies, and a composer must decide each voice's place in the melody and harmony. Voice leading refers not only to singing voices, but also to the voices of the different melodic instruments within a piece. It can be used for two or more instruments, though harmonies used in this type of composition most often include three-note chords rather than two-note intervals. Baroque music is one type of music known for using voice leading techniques to establish melodies and sub-melodies within a harmonic structure.
Though they are generally written with the root at the bottom for study purposes, in practice, chords can be rearranged and modified to fit in music. When a composer works with a set harmonic structure, she can use voice leading to determine how these chords should best be represented within the piece. The composer can use this technique to determine which notes to include, which to highlight, and which to drop.
This technique is nearly ubiquitous in basic music composition courses. An accepted method for teaching voice leading is called counterpoint. It is a common feature of tonal music, in which the chords of a piece are defined by modal root progression, and voice leading is used to choose how the chord pieces will sound out within a musical composition.
Chords have basic notes that make them unique, and good voice leading can help ensure that these notes ring out. When using a triad, part of good composition and arrangement includes voicing the parts of a chord that set it apart from other chords like it. For instance, if the chord in the music is a minor triad, which is made up of a major root, a minor third, and a fifth, the composer should choose to voice the root and minor third to make the minor elements of a chord come through. If the composer were to voice only the root and fifth of the chord within a multiple instrument composition, it would be indistinguishable by ear whether the chord was minor as intended, or a major fifth or major triad, which use similar notes.
In addition to theoretical guides that help composers with voice leading, it can also be important to consider the instruments used in the piece. When writing music, the composer must consider not only what sounds good, but also what the singers and instrumentalists are going to be able to play. Each instrument has a range of notes in which it is usually played. Writing notes outside the usual range of an instrument can make a piece more difficult to play, requiring the instrumentalist to have a special skill for notes outside of the normal range of the instrument. Unless intentionally trying to achieve the sound of an instrument played outside of its normal range, many composers choose to switch instruments or transpose notes when a melody falls outside the playing range of an instrument.