Organum is the name for two or more voices that share the same rhythm and move by the same interval.
I modeled this next piece after Russo's Example H in Composing Music, pg. 124-125.
Measures 1-8 make up the small theme, which is divided between the clarinet and the bassoon.
Measures 9-10 show the bassoon in perfect fourths below the melody.
Measures 11-12 show the clarinet in perfect fifths above the melody.
Measures 13-14 show an inexact organum of the lower sixth in the bassoon.
Measures 15-16 show the clarinet a minor third above the melody.
Measures 21-22 are in unison.
Measures 23-24 are in octaves.
Measures 27-28 are in unison.