This remained the basic element in Western harmony until the 20th century, even when harmony was composed in four parts or more. The study of harmony also dictates acceptable relationships between successive chords. The cello plays the melody and piano the chordal accompaniment. You know you can sing the melody against itself. This technique is called imitation, which is a type of polyphony.
Strophic form is a form of vocal music in which all stanzas are sung to the same music.
This form is very old and is still used today in all forms of popular and church music. There are other forms as well. We will talk about them as we encounter them. Orchestration is how the composer uses those instruments and voices.