It is a square in a square but using a rectangle instead. Probably the accent border is making the length to be divisible by the size of the block. Thanks for posting. I might just do this border for a baby quilt I have been trying to figure out an easy way of piecing a border.
This at least gives you some other blcoks to make up the border:
http://colorgirlquilts.com/2015/02/a...nd-border.html