Fabric grain refers to the direction of threads woven into fabric. Fabric is woven using two or more threads at right angles to each other, creating two grains - lengthwise [warp] and crosswise [weft]. The warp runs up and down the length of fabric, while the weft is woven over and under the lengthwise grain from side to side. A selvedge is created as the weft threads turn back around the edges to continue weaving in the opposite direction. This creates a continuous binding on the length of the fabric.
These grain lines react in different ways. The warp thread, or lengthwise grain, is the stronger of the two and less likely to stretch out of shape. There are more warp threads per square inch than there are weft threads. These extra threads create strength. The lengthwise grain is the grain most used in garment construction.
The weft, or crosswise grain, is less dense and has some give because of the over and under weaving. Most garments use the crosswise grain around the circumference of the body, giving the garment an inherent give where needed.
Related, but separate to these two, is bias grain. The bias grain refers to a direct line at a 45 degree angle to the crosswise and lengthwise grain. Using bias grain creates softness and a drape.
The fabric’s straight grain will naturally hang perpendicular/parallel to the floor, no matter how the pattern pieces are cut out. The larger or longer the pattern pieces the more careful you need to be in cutting your pieces "on grain." If they are off, the drape of the garment may be affected - i.e. you pant leg may twist around the leg at the hem. How do you make sure your fabric is on grain...and what do you do if it is not?
Straightening the Grain
After pre-washing your fabric you may find, as I did, that you need to re-straighten [block] the grain. You can tell if the grain is off if the fabric’s selvedge edges do not match when folded.
Rip the cross grain on one end to ensure you are starting with the cross grain edges straight and matching.
Fold your fabric in half with ripped cross grain and selvage edges matching.
Observe which one of the corners is short. Hold the short corner with one hand and with the other hand, grasp the opposite corner. Gently stretch the fabric on the diagonal, moving your way down the length of fabric - continuing to stretch.
Fold it in half again to see if the edges now align. Repeat the gentle stretching if necessary. If using a fabric with a printed design, be careful not to stretch too strenuously or the design motif can be stretched out of shape.
So there you have it, everything you ever wanted to know about fabric grain but were afraid to ask. (Well, maybe not everything…) Now that you understand the difference between the three types of grain and how they should be oriented in your fabric, hopefully you won’t have any trouble fixing misaligned grainlines from now on. And if all else fails, remember that a little bit of stretch goes a long way- just be careful not to overstretch your fabric or you may end up with some unwanted distortion. Have any questions? Feel free to leave them in the comments section below and I’ll do my best to answer them. Happy sewing!