Quotation from: The Notebooks of Leonardo Da Vinci

Written by: Leonardo da Vinci

Domes rising from an octagonal base.

This system, developed according to two different schemes, has given
rise to two classes with many varieties.

In a) On each side of the octagon chapels of equal form are added.

In b) The chapels are dissimilar; those which terminate the
principal axes being different in form from those which are added on
the diagonal sides of the octagon.