A Dictionary of Music and Musicians/Fa Fictum
FA FICTUM. In the system of Guido d'Arezzo, B♮, the third sound in the Hexachordum naturale was called B mi; and B♭, the fourth sound in the Hexachordum molle, B fa. And, because B fa could not be expressed without the accidental sign (B rotundum) it was called Fa fictum. [See Hexachord.] For this reason, the Polyphonic Composers applied the term Fa fictum to the note B♭, whenever it was introduced, by means of the accidental sign, into a Mode sung at its natural pitch; and, by analogy, to the E♭ which represented the same interval in the transposed Modes. The Fa fictum is introduced, with characteristic effect, in the 'Gloria Patri' of Tallis's five-part Responses, at the second syllable of the word 'without'; and a fine example of its employment in the form of the transposed E♭ will be found in Giaches Archadelt's Madrigal, 'Il bianco e dolce cigno,' at the second and third syllables of the word 'piangendo,' as shown in the example in vol. ii. p.188 b.
[ W. S. R. ]