In two-part writing the primary intervals used are 3rds and 6ths, with some Perfect 8ves and 5ths. For dissonances or Non-Chord Tones, use 4ths, 2nds, and 7ths. Interplay between the voices and textural variation are important features.
In three-part writing there is more flexibility. A third part can join the first two to complete sonorities, or it may provide a pedal point or ostinato.