The forms of American march music typically are of three categories: the military march form, the regimental march form, and a general group containing recapitulation marches, "four-step" marches, and other diverse forms. All marches have at least three common elements, including: different (i.e., contrasting) sections called strains; several different melodies; and a "trio" section of strains/"repeats" that offers pronounced contrasts in phrasing. Most American marches use (seemingly) simple chord progressions, but—using chromatic harmonies, sevenths extensions, and secondary dominants—composers often complicated their marches with interesting chords and rapid chord changes.