A mode in species modal counterpoint is a succession of 8 traditionally white notes (i.e. notes that do not contain sharps or flats) in an ascending or descending order, beginning and ending on any white note except B. The mode of a cantus firmus in species counterpoint defines the melodic range of its counterpoint.
For each of the six possible notes to start and end on there are two possible modes. The first is the authentic ambitus which begins on a note and ends on the same note. The second is the plagal ambitus which begins on a note and ranges from the note a fifth above to a fourth below, these modes carry the prefix, hypo, and are considered to be different modes. Starting on C (Ionian), there are 12 modes in traditional modal species counterpoint:
Ionian, Hypoionian, Dorian, Hypodorian, Phrygian, Hypophrygian, Lydian, Hypolydian, Mixolydian, Hypomixolydian, Aeolian, and Hypoaeolian.