This fantasia is a monothematic cantus firmus composition based on a five-note theme fa–mi–fa–re–ut (F–E–F–D–C). The theme is stated twice in the title but never presented as such in the work. It is probably given twice to show that the position of the semitone can be variable, either fa–mi–fa–re-ut or sol-fa–sol–mi–re. The piece breaks into two distinct sections of equal length: compases 1-27 + 28-60. The theme is constantly present except following the cadence in bar 20, ie bars 21-22. In the first half of the work the theme is treated as the basis for imitative polyphony. In the second, it is used as an ostinato, on C and G in the sequence A-B-T-S-B-T-S.

First half: exposition of version 1a of theme in b. 1-12. Version 1b of theme is treated in b. 12-27. This time ina seemingly full 4-voiced texture with entries at the 5th. There are 2 successive statements in the superius of the theme and a hint of a third, but Mudarra dashes expectations and leads the voice off freely in another direction. The full close at b. 27 marks the end of the first type of thematic treatment.

Second half: The new texture has predominantoly only 2 sounding voices at any given moment. -- the ostinato in semibreves and the decoration in semiminimas. The first four statements of the theme are almost made into voice pairs given their decorative accompanying voices. The theme is stated A-B-T-S-B-T-S. At b. 45 there is a new direction as the decorative pasrt bcomes entirely free and the ostinato is heard B, T, S -- again saving the treble statement for last.

Venegas de Henestrosa reworked this fantasia. See 1557/2, no. 73 [Anglés edition, p. 109, Fantasia XIX]