Want your finished garments to hang beautifully? Of course you do! Don't pick up your scissors yet . . . you need to first understand grainline, and know how to straighten it for beautiful results!
Woven fabrics need to be straightened before cutting. Picture the threads forming a large grid pattern - some going lengthwise and some going widthwise.
First let's define grainline. Grainline is the direction the threads are running. It's important to understand because how you cut out a garment will change how the finished garment behaves. More on that in another post.
There are three grains: straight grain, cross grain, and true bias. Straight grain is the threads going parallel to the selvedge of the fabric - the uncut edges that are bound so that they do not unravel. When fabric is cut at a shop, it is cut on the crossgrain. The crossgrain is the direction of the threads running the width of the fabric - from one selvedge to the other. It is perpendicular to the straight grain. If you picture the straightgrain being the longer threads that run in a straight line the entire length of the bolt of fabric and the crossgrain being the shorter threads that run across the width of the fabric, you'll remember the difference.
In a properly straightened piece of fabric, the straight grain and the cross grain should run at right angles to each other. However, sometimes fabric comes off the bolt warped from storage. If this happens, it is sometimes enough to pull on opposite diagonal corners until the fabric straightens itself. At times I've had very warped fabric that I had to get wet and reshape while damp. This can be tricky but luckily is usually not necessary.
Why is all this important? It affects how the garment will hang. To understand this, let's define bias. True bias is the 45 degree angle between the cross grain and the straight grain. To really get it, take any small square of fabric that is cut on grain. First pull on opposite edges. You'll normally see the cross grain edge straight just slightly (assuming this is a non-stretch woven), and the straight grain stretch not at all. This is why, for example, waistbands are cut on the straight grain, not the cross grain. You want them to be stable and not stretch out. This is also why pants are cut so that the straight grain runs vertically, not horizontally - because you want the fabric to have give and ease across the width of the pants rather than up and down. Now pull on opposite corners of the square. You'll see for some fabrics a medium stretch or shift in the fibers, and in some fabrics a dramatic shift in which the square no longer is a square. This is why bias cut dresses drape on the body in such a flattering way. It is also why they are harder to sew.
Now imagine a garment in which the pieces were cut not on the straight grain, not on the cross grain, and not on the true bias, but just slightly askew (slightly off grain). What would happen? The fabric would sag and pull slightly in the direction of the diagonal, making for an unflattering finished garment.
Therefore straightening the edge of grain is a very important, even if a bit tedious, task to do before cutting out pieces. Here's how to do it:
Assuming your fabric is not warped (if it is, pull on opposite corners to straighten it), take one small perpendicular snip into the selvedge near the cut end of the fabric - about two or three inches from the end, depending on how diagonally the fabric was cut (some shops cut it extremely straight, others, not so much). From your snip you should be able to gently fray the threads so that they come loose. Take a thread that runs right down the center of the snip - and gently pull with your fingers. As you pull it, the cross grain thread should pucker the fabric. Move these puckers very gently with your thumb and forefinger while keeping tension on the thread with your other hand. Don't try too much at once or it will break. Pull up at least several inches of gathers, or more if you can. Flatten out the gathers by pushing them forward so that it's more of a slight ripple in the fabric. Now cut along that ripple until you come to the end of the ripple (try not to cut the thread you just pulled - cut just to one side of it). Then pull it gently again and cut some more.
The purpose of pulling the thread is so that you can directly see one isolated crossgrain thread. This allows you to cut across the fabric using one thread as a guide so you wind up with a perfectly straight edge. Some fabrics have sturdy enough threads you can make it all the way to the opposite selvedge without the thread breaking, but this usually doesn't happen. If/when it breaks, simply take another thread as close as possible to the thread you were working with, and continue on.
Once finished, you will have a perfect edge. From here it's even more obvious if your fabric is straight. If you lay out your fabric and notice that the grainlines do not match the grid pattern on the cutting board (i.e. it's not perfectly square), again, gently pull opposite diagonal corners and smooth the fabric with your hands until it is on grain. If it can't be straightened, a blast of steam from an iron usually does the trick, and if even that doesn't work, at this point I will dampen it and reshape it and let it dry. An unfortunate detour, but much better than spending a lot of love on a garment and winding up with a crooked finished piece!
There: that wasn't so bad, was it? Now your garments will be cut perfectly straight and will hang beautifully. Hurrah!
Next step: cutting on the grainlines.