An ostinato fantasia of 123 compases based on the solmised figure ut, sol, sol. la, sol. The ostinato is designated “passo forçoso” in the work’s rubric, and as a “passo forçado” in the table of contents . As title indicates, ostinato only occurs in bass part. (or lowest, see opening). Bartolomé Cayrasco de Figueroa (1538-1610).Templo Militante , triumphos de virtudes, festividades y vidas de santos. (Valladolid, 1602-1603). refers to “passos forçados” and “passos forçosos”. The relevant passages are given in the entry for Cayrasco’s work in the section on documents.

The structure of the piece may be seen in the treatment of the ostinato. (M=minim, C=crotchet)
I (1-61) Intermittent appearances of the ostinato in the rhythmic form: |M|CC|M|M|
II (61-72) the ostinato is shortened to C|CC|C with one G omitted. = K to all before it.
III (72-100) the ostinato is augmented to the pitches F–C–D–C (2 minims on each note), with theme 3 in imitation
IV (100-119) the ostinato is used as at the opening.
V (119-123) – homophonic Coda.

Opening of work has S/T – A/B pairs of theme 2 against theme 1. Theme 3 is imitated against the augmentations of theme 1. Except for the penultimate & anti-penultimate thematic entries which begin on Bb, all begin on F. – a total of 16 appearances. Free inventive part-writing pervades the rest of the work. Harmonically rich work. Theme 3 announces a more idiomatic style.

Proportionality is also evident in the work motion too: Section I ends at 61 - the midway point of the work.