The Hypoionian mode, literally meaning 'below Ionian', is the name assigned by Henricus Glareanus in his Dodecachordon (1547) to the plagal mode

Musical mode

In the theory of Western music since the ninth century, mode generally refers to a type of scale. This usage, still the most common in recent years, reflects a tradition dating to the middle ages, itself inspired by the theory of ancient Greek music.The word encompasses several additional...

 on C, which uses the diatonic octave species

Octave species

In early Greek music theory, an octave species is a sequence of incomposite intervals making up a complete octave...

 from G to the G an octave higher, divided at its final, C. This is the same as playing all the white notes of a piano from G to G: G A B C | (C) D E F G (Powers 2001, 37).

Glarean regarded compositions with F as the final and a one-flat signature as transpositions of the Ionian or Hypoionian mode (depending on the ambitus

Ambitus

In ancient Roman law, ambitus was a crime of political corruption, mainly a candidate's attempt to influence the outcome of an election through bribery or other forms of soft power...

). Most of his contemporaries, however, appear to have continued considering such compositions as being in the fifth and sixth modes (Lydian

Lydian mode

The Lydian musical scale is a rising pattern of pitches comprising three whole tones, a semitone, two more whole tones, and a final semitone. This sequence of pitches roughly describes the fifth of the eight Gregorian modes, known as Mode V or the authentic mode on F, theoretically using B but in...

 and Hypolydian

Hypolydian mode

The Hypolydian mode, literally meaning "below Lydian", is the common name for the sixth of the eight medieval church modes . The name is taken from Ptolemy of Alexandria's term for one of his seven tonoi, or transposition keys...

), which had been regarded since the beginnings of medieval modal theory as preferring B over B for the fourth degree above the final, F (Powers 2001, 37–38).