Why do some grain silo's in Iowa have checkerboard patterns around the top?

There used to be (I don't know if they are still in operation) a company called "Madison" which used to build silos out of a special shape of concrete block. The trademark checkerboard pattern at the top was simply a decoration. The workers just painted a few of the blocks white before construction, then when they got to the top two courses (or rows) of blocks, they just slipped those white ones into every other slot. The crews were very experienced, and had the ability to erect a 25 foot diameter, 80 foot high silo in a single day. A Madison silo was a very stable and long-lived structure and usually cost much less than the big blue ones ("Harvestore" brand). They were even known to have survived smaller tornados.

See the related link below to see a photo of a couple Madison silos next to an old barn.