Pomeroy (2004), BaileyShea (2007), and Nobile (2020) have expanded the concept of the “double-tonic complex” (DTC) since Bailey’s (1985) coining it. While Pomeroy and BaileyShea discuss two tonics related by chromaticism or mode mixture, I argue that the theory of DTCs possesses considerable power to explain the tendency of diatonic modal music to exhibit multiple tonal centers (Lam 2020). I expand upon Nobile’s formulation of the Aeolian–Ionian complex to a generalized concept of a modal DTC. In addition to the Aeolian–Ionian complex, I propose Ionian–Phrygian and Dorian–Lydian complexes and explore how dual tonal centers are interwoven within these relational structures.
Three pieces by Maurice Duruflé—the “Introit” from the Requiem, Op. 9; the Prélude sur le nom d’Alain, Op. 7; and the “Pie Jesu” from Op. 9—serve as a case study in which five musical conditions manifest modal DTCs. These conditions are as follows:
1) Plainchant priority: the final of the plainchant or plainchant-inspired melody defines one of the tonics.
2) Tonally ambiguous common-tone chord: a prominent seventh chord or first-inversion triad containing both tonic pitches can refer to both tonics.
3) Multiple harmonizations: different harmonizations of the same melody point to either tonic, inviting hearings of the melody in the context of both modes.
4) Multivalent points of imitation: a motive is transposed to reflect the same scale-degree succession in both modes, or alternatively, a motive is preserved in pitch yet harmonized in both modes such that the same motive is ultimately heard in the context of both modes.
5) Rhetorical-formal significance of beginnings and endings: even if one tonic or the other attains more salience at various middle points, the opening or closing tonic is understood to be particularly significant.
These conditions show how a modal DTC represents neither directionality nor tension but, rather, a mutually reinforcing expression of the music’s distinctive modal character. While these particular conditions might be unique to Duruflé’s works, future research can explore how the modal DTC emerges in other diatonic modal music by composers ranging from Vaughan Williams to Stravinsky.