There are times when a piece of fabric yardage must be square or true in order for a length of fabric to drape or hang properly without twisting. If a reference edge, like the selvage edge, is no longer attached to the fabric piece then it is possible to find the true grain of woven fabric once again.
Finding the true grain of fabric is important in drapery sewing, when laying out pattern pieces onto fabric, for pleats to hang straight, stripes to not twist, for printed fabric designs and surface texturing to appear balanced in their patterning and for facings and linings to properly support the fabric without meandering to one side.
The terms warp and weft are associated with the process of weaving yarns, spun fibers or threads into fabric. Weft threads are drawn under and over parallel placed stretched warp yarns to create a woven fabric. All woven fabrics are constructed with warp and weft threads.
Originally wool, flax, silk or cotton was used to weave into fabric. Today, completely synthetic fibers or a combination of natural and synthetic fibers are predominately used in weaving on automated industrial looms although hand manipulated wooden looms may still be found.
Make sure to wash, dry and iron the fabric, if the fabric care allows, to get rid of finishes and any wrinkles that can artificially misrepresent the grain.
If you still have the selvage edge attached to at least one side of the fabric then it is easier to square up the fabric piece - the selvage is the uncut, finished edge of the fabric which is on the right- and left-hand edges of the fabric as it comes off the loom.
Starting with a selvage edge facing you at the top, fold the fabric in half so the selvage is folded over itself. Mark the crease in the fold with a pencil or piece of tape in the selvage edge fold at the top edge and at the opposite bottom edge of the fold, in the crease. Open your fabric and find a thread along or near-to this marked crease that you can pull out or lightly chalk along its length. You will now have the selvage at the top and the missing thread line pulled out or chalk line to use as two of the squared edges. Parallel and opposite the selvage and parallel and opposite the missing or chalked line would show the true square of the fabric.
If there are no selvages to follow, then open the fabric if folded and find two long threads near-to the cut edges that appear to intersect at some point. Pulling out the threads that appear to be perpendicular to each other would help you find the warp and weft threads that would intersect. Using two yard sticks set perpendicular to each other would help if pulling a long thread seems too difficult. Just look carefully at the threads along their lengths to determine where to position your yardsticks. Mark the true grainline with pins or a chalk marker before you cut your pattern pieces or fabric lengths to be sure your placements are on or parallel to this true grain.
If after finding the perpendicular weft and warp threads your fabric still seems to twist a bit, then take hold of the fabric at opposite sides and pull at intervals along the length of the fabric. Repeat the process if necessary. Insulated-backed fabric cannot be squared and is presumed to be on grainline.
Some inexpensive printed fabrics may not be imprinted true to the grainline of the fabric; squaring this then is not feasible. This may be more noticeable close to the selvage edge; it is best not to use the selvage edge when cutting out pattern pieces. Once washed, some fabrics may find the selvage edge puckering as the close under/over weave of the fabric edge may shrink, however the selvage edges have unique uses of their own.
Sew happy, sew inspired.