I believe, that the ribs of the corduroy are the wales. they run the straight grain of the fabric. (the same direction as the selvages, NOT from selvage to selvage). So I believe what they are saying is to cut the squares so that the wales run straight in the block not in any way slanted. In other words you would line up the edge of your ruler with the wales. Does this make sense? If you line up the edge of your ruler that way, the other edges of your block are automatically perpendicular. I don't know why they added that, it just confuses things. They just want your blocks not to be on the bias, even a little.