Polyphony is a characteristic of Western music. In its early form, polyphony was based on plainchant. It began when singers started improvising with parallel melodies, with emphasis on fourth (ex. C to F) and fifth (ex. C to G) intervals. This marked the start of polyphony wherein several musical lines were combined. As singers continued experimenting with melodies, polyphony became more elaborate and complex.
A round is a vocal piece wherein different voices sing the same melody, at the same pitch, but the lines are successively sung. An early example of a round is Sumer is icumen in, a piece that is also an example of a six-voice polyphony. The children's song "Row, Row, Row Your Boat" is an example of a round.
A symphony often has 3 to 4 movements. The beginning is moderately fast, the next section is slow followed by a minuet and then a very fast conclusion. Symphonies had its roots from Baroque sinfonias but composers like Haydn (known as "The Father of the Symphony") and Beethoven (whose popular work includes the "Ninth Symphony") further developed and influenced this music form.