Quatrefoil Window in La Jolla Church

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.