Use of Accidentals in 16th-Century Style
The only accidental available in the hexachordal solmization system used by singers in the 16th-century style was B-flat. It was used in the so-called "soft" (molle) hexachord beginning on F (as opposed to the durum or "hard" G hexachord with B-natural) to "soften" the tritone between F and B (and also to "soften" the upper neighbor to A).

By the 16th century the B-flat had become standard notation in most cases, but many of the other half-step alterations that had become standard singing practice were not generally given in the notation. The extent to which this "false music" or musica ficta was employed in performance can never be determined; however the practice of raising the subfinalis (creating a leading tone) at cadences is generally understood to be the basic usage of musica ficta. In order to somewhat preserve the original notational practice, accidentals for musica ficta are generally given above the notes which they alter. The only notes to which musica ficta can be applied are F, C, and G.

Note: F-sharp (B-natural in transposed modes) is never used to correct the tritone.

The following chart outlines acceptable use of accidentals in 16th-century style. Remember that musica ficta is only used at cadences (except in familiar style):

Mode Use of B-flat Allowed Musica Ficta
Dorian Often C#
Phrygian Rarely None (F-natural is an
upper "leading tone")
Lydian Very often None
Mixolydian Sometimes F#
Aeolian Sometimes G#, (F# for fusas only)
Ionian Sometimes None
Transposed Modes Use of E-flat Allowed Musica Ficta
Dorian Often F#
Phrygian Rarely None (B-flat is an
upper "leading tone")
Lydian Very often None
Mixolydian Sometimes B-natural
Aeolian Sometimes C#, (B-natural
for fusas only)
Ionian Sometimes None
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