I really love these windows. They are common enough around California
that I realized there must be something behind the design. This one
is from a church in La Jolla, near San Diego.
After a little poking around Google I found out that this four sided
circle design has a long history in Moorish Architecture and Celtic design.
It is called a Quatrefoil. [Quatre: "Four" in French, foil:
Old English meaning "leaf"]. Not a large leap then, once you know the root of the
word, to realize why these designs are common.