I respectfully disagree with Thusnelda. Six inch blocks finish to 5.5. So the middle square on the D9P will be 5.5. The top and bottom squares will only use ONE seam allowance each at this point(where they are sewn to the middle square) so at this stage they will be 5.75 inches. So now you have 5.5 inches plus 5.75 plus 5.75 which totals 17 inches. Too long for her panel. She needs to put a border around her panel in order for it to accept these D9P blocks. In Thusnelda's math above, 18" minus 4 seams at .25 each DOES equal 16.5 but this does not account for the top and bottom seam allowances of the D9P, only the four seam allowances used to sew the top and bottom rows together. So the D9P at this stage will be 17 inches.