principal axes.

The elevation, drawn on the same sheet (see page 47 Fig. 3), shows
the whole arrangement which is closely related with the one on Pl.
LXXXVI No. 1, 2.

MS. B. 21a shows:

a) four sides with rectangular chapels crowned by pediments Pl.
LXXXVII No. 3 (plan and elevation);

b) four sides with square chapels crowned by octagonal domes. Pl.
LXXXVII No. 4; the plan underneath.

MS. B. 18a shows a variation obtained by replacing the round chapels
in the principal axes of the sketch MS. B. l8a by square ones, with
an apse. Leonardo repeated both ideas for better comparison side by
side, see page 47. Fig. 2.

Pl. LXXXIX (MS. B. 17b). Elevation for the preceding figure. The
comparison of the drawing marked M with the plan on page 47 Fig. 2,
bearing the same mark, and of the elevation on Pl. LXXXIX below
(marked A) with the corresponding plan on page 47 is highly
instructive, as illustrating the spirit in which Leonardo pursued
these studies.

Pl. LXXXIV No. 12 shows the design Pl. LXXXVII No. 3 combined with
apses, with the addition of round chapels on the diagonal sides.

Pl. LXXXIV No. 13 is a variation of the preceding sketch.