I have Carrara 6 Pro, could not find any books for it, so I bought the one shown in the image above. 

On page 7 the book states "A polygon is a type of geometric object that can either be opened or closed."

The book doesn't elaborate on it at this point, so can someone please show me what they mean by an open polygon?

Yank My Doodle, It's a Dandy!