Leta, my guess when I looked at this was they started with a 10" square layer cake. I was going to cut off 2" from 2 adjacent sides, then use 1.5" strips for the black separating portion, adding small black pieces to add enough length to the cut off layer cake strips to make a 10.5" unfinished square (10" finished). Then sash with 2.5" sashing. I think these measurements will give a good approximation of the proportions in the photo.