A chord is made up of at least three notes, called a triad. With voice leading, all of the voices -- blocs of chords -- move in the same direction up or down the musical scale, or in different directions, depending on the way the music is written. When you learn voice leading, it is presumed that you already know something about music theory and how intervals, triads and four-part harmony work.
In writing music for a voice lead, four types of motion exist. Parallel motion occurs when voices move in the same direction at the same pace. Similar motion occurs when voices move in the same direction and the pace varies. Contrary motion occurs when voices move in the opposite direction. Oblique motion occurs when one voice stays in one place, and the other voice moves up or down the scales.
Though no real "rules" for voice leading exist, the principles will involve tone, continuity and pitch. The sounds of music evoke in the human mind certain images and moods. Auditory images tell music listeners what a lead is and how it differentiates from the background music. Even those who do not understand the concept of music can hear how the lead varies from the supporting parts or instruments.
Creating four voices in choral style involves learning to control each voice and understanding the relationships among the four voices. The technique of controlling these voices has two goals: establishing independent voices and maintaining clear tones during voicing. All voices form melodies, and the outer voices carry the prominent melodies, or leads. It is not a good idea to make drastic chord changes when writing music for voice leads.
Voices that move in an independent way rather than in a parallel or contrary motion sound the best when voicing leads. Voice leading uses steps, leaps, intervals, movement and elaboration in order to make the chord progressions make sense and sound good. Any voice or instrument can become a "lead" in a piece of music.