Curtain fabric is made of two sets of threads. One positioned horizontally, across the fabric from selvedge -- the tightly woven edge -- to selvedge called the crosswise grain. The other, positioned vertically, up and down the length of the fabric, is the lengthwise grain. Curtains and drapes will not hang straight if you don't cut the fabric straight. Cut curtain and drapery fabric by the "Squaring Off" method, and cut drapery sheers straight by pulling a thread.
Cut the selvedge edges off. Lay the fabric flat, right side up, on a large cutting surface. Position the cut edges to your left and right. The bound edges, or selvedges, run along lengthwise grain of fabric and are toward you and across the table the width of the fabric. Determine the width of the tight weave of the selvedge where the selvedge tightness changes to the weave of the fabric. Mark this width all along the lengthwise edge of the fabric and cut on this line.
Lay the inside edge of the long end of a carpenters square along one lengthwise edge of the fabric. Draw a line along the outside edge of the short arm and extension on the square from one lengthwise edge of the fabric to the other, using a 54-inch ruler to extend the other arm of the square. Cut on this line.
Measure from this line along the lengthwise edge to the required length and mark the fabric. Measure from the halfway point of the first cut line to the required length and mark this point. Measure again on the remaining lengthwise edge and mark at the required length. Connect all three marks and cut on this line.
Grasp one exposed thread on the lengthwise edge of the fabric and gently pull, to remove a thread on sheer fabric. The fabric will gather and the pulled thread will get longer. If the thread breaks, lay the fabric down, straighten out any gathers and cut on the line left visible from the pulled-out thread. Continue pulling a crosswise thread and cutting on the line created by removing the thread.
Curtains and drapes will hang best if the fabric is cut straight to the lengthwise edges. Do not rip drapery fabric; the edges will be forced out of aligment and the result will not be square.