Learn Counterpoint here: https://www.mymusictheory.com/learn-music-theory/reference/579-counterpoint
Counterpoint is the art of weaving together independent melodies in order to produce a beautiful, harmonious whole. Each part is tuneful and interesting in itself, and when parts are combined with each other, we hear the result as harmony. The music then, has both a horizontal and a vertical aspect.
A well-known example of a simple kind of counterpoint is the round “London’s Burning” – each voice sings its own individual line, but because the voices begin at staggered time intervals, different notes overlap with each other, creating harmony. This technique is called “canon”.
At the other end of the spectrum, some of the most complicated counterpoint can be found in fugues.
Fugues begin in the same way as rounds – with the melodic idea that’s heard at the beginning being introduced by different voices at staggered time intervals. But in contrast, in a fugue, the second entrance begins in a different key (usually the dominant key: for example, if the fugue begins in C major, the second entrance would be in G major), and the melodic material is then adapted and developed in clever and interesting ways.
We use the terms “voice”, “line” and “part” to mean the same thing – an individual tune. It could be sung, or played by an instrument. In “London’s Burning”, we would expect sung voices of course. In a fugue written for keyboard there is only one player and one instrument, but the effect of different “voices” is still there.
Bach’s Fugue no. VI is written for three “voices”, for example.
“Voice 1” is in the treble clef stave, with stems up. “Voice 2” is split between the staves. It begins with the stem-up D in the bass clef, then moves to the low-position rests in the treble clef. It then continues in the treble clef, with the stems-down notes. “Voice 3” is in the bass clef, starting with the rest, then continuing with stems down. Voice 3 has the original melody that we saw earlier, this time in the home key of D minor again.
We usually refer to the voices as soprano, alto, tenor and bass, depending on their general overall pitch, regardless of whether they are played or sung.
There are two adjectives to describe counterpoint; “contrapuntal” and “polyphonic”.