The sounding of two or more musical notes at the same time in a way that is pleasant or desired is called .

Harmony. 
rhythm is to do with beat, nocturne is a short composition, melody is tune. harmony is, for example, singing a slightly different melody to the main melody which fits in with the main melody.

The gregorian chant was a major cultural milestone in the middle ages. it was one of the first codified forms of music meant to be played as written, where previously it was difficult to mass-produce and disseminate any music. what's more, these chants were also some of the first examples of musical notation: the notes and bars of today's music sheets are a reflection of centuries of evolution beginning with these chants. it became official throughout the entirety of the catholic church, meaning that nearly all of europe followed this musical notation and styling.

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