Hi Michelle, What I do is put any size sashing that I think looks nice between the blocks. It dosen't matter the width, as long as the sashing is the same width between the horizontal blocks as well as the vertical blocks.. I like the sashing to be 1 3/8" to 1 1/2". Don't forget to add 1/4" to the finished size. I don't like the sashing to be too wide as I think it takes away from the "borders"...Suzanne from Mich