Counterpoint
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Counterpoint is the art of composing music by combining different parts (voices) in a way that sounds nice. Music composed like this is called contrapuntal.
If you play the tune of Twinkle, twinkle little star on the piano and then add some chords, this is harmony, not counterpoint.
But now try a different way: start the tune with your right hand. In the second bar (on the fifth note) start playing the tune in the left hand an octave lower. This works well for a time, but in the fifth bar (on the word “Up” in the right hand part) it starts to get crunchy, and you could make changes to the left hand to make it sound nicer. This is contrapuntal music.
In that example the left hand imitated the right hand at first. This is called imitation.
If the second part had continued to imitate all the way through the piece it would have been a canon. But “Twinkle, twinkle” does not work well as a canon. One famous canon is by Thomas Tallis. A canon that can be repeated is called a round. This is all contrapuntal music.
Counterpoint does not have to have imitation, although it often does. The important thing is that each part (i.e. each voice) is equally important. It is not one part singing the tune and the rest just accompanying.
Counterpoint does not have to be one note against one note. There can be two or more notes in one part against one in the other. There is a whole system for this called “species”.
Counterpoint can be varied by inverting it, i.e. putting the top part at the bottom. When music is written so that the parts can be swapped round it is called "invertible counterpoint".
The word “counterpoint” comes from the Latin “punctus contra punctum” meaning “point against point”. The word “point” meant “note”. Several hundred years ago composers found how write contrapuntal music. They often took a main tune (called a “Cantus Firmus”) and then added one or two or more parts to it. The more parts there were the harder it was to compose because it all had to fit. Music for several voices written in this way is called polyphonic. Polyphony was used in all church music in the Renaissance. The greatest composer of polyphony was Giovanni da Palestrina (1525-1594). Students learning the art of composition still learn counterpoint today by taking Palestrina’s music as their model.
See also: Fugue