Hello Ladies (and gents)

I'm having a problem and asking for help.....I'm trying to make a quilt without a pattern except for in my head and down on scratch paper..... what i'm trying to do is make squares 9", one will be a patterned piece of material and the other ones on each side of that will be a 9" but will be 4 triangles of two different colors, sorta like making a frame around the printed fabric.so like the top row in my quilt would be a 9" patterned piece, 9" 4 triangles, 9" patterned piece, 9" 4 triangles so I end up with 6 - 9"squares then just the opposite in the next row......but my delima is -- when I cut my fabric for the 4 triangles -- I cut two pieces 9" each sew them to cut into 4 triangles and then sew them..and my square comes out much bigger than 9"....so the QUESTION is - what size do I cut those two pieces of material to cut them in triangles and then sew them together to come up with a 9" square.... appreciate all and any help......thank you so much wow I hope someone understands me.... LOL