A Dictionary of Music and Musicians/Subdominant
SUBDOMINANT. The fourth note of the scale upwards. The note below the dominant, as F in the key of C. The radical bass of the penultimate chord in the Plagal cadence. When groups of movements are balanced together in threes the central one is most frequently in the key of the subdominant, as in sonatas of three movements, the minuet and trio form, marches, valses, etc. In the actual body of a large movement in forms of the sonata order, the key of the subdominant is not antithetically acceptable, and examples of its occurrence in modern music as the key of the second section or second subject are extremely rare, and evidently not well advised. But in dependence on the tonic key it is one of the most important of harmonic centres, and digressions in that direction are very common in modern music.
[ C. H. H. P. ]