Because I alway miscut fabric, to long or too short. I use the lay down a single strip and cut to size as I proceed around the square. If you are using a foundation square, it might be easier. And although it might seem like a time consuming first step, make yourself a template of the actual square you want to make and keep it handy. It will be you diagram. If you are using a foundation piece draw line from corner to corner and where the x's meet is where you place you center block. it is the start.