I remember my mom trying to explain the grains of fabric when I was first learning to sew. Of course, a 6 year-old couldn't have cared less what "warp", "weft", and "bias" meant. All I heard was "blah, blah, blah, look at all the big words I know!". :o)
But it's simple, really! And sometimes can be very helpful to understand. Fabric is woven with a bajillion little tic-tac-toe threads, running across each other.
- "Warp", or "lengthwise grain", is just the name of the threads in the fabric that run up and down, alongside the selvage (another scary word, which is just the white part at the edge of the fabric, usually with the brand on it).
- "Weft", or "crosswise grain" are the threads that run perpendicular to the warp threads
- "Bias" is the term for the where the threads meet diagonally.
This can come in handy to know, because all fabrics, even non-stretchy ones, stretch a little bit.
This pink fabric is just a plain cotton, and when I stretch the warp threads (photo 1), there isn't really any stretch at all. When I apply the same stretching to the weft threads, there is a little give (photo 2), but when I stretch it on the bias, it's very stretchy! Sometimes you want a little stretch in the piece you're cutting, and sometimes you don't. For example; when making bias tape, you want to cut on the bias, for optimum stretchiness, since it will be stretching around the corners of a garment! :o)
The first thing you want to do with your fabric is even up the top and bottom. They can't always cut fabric perfectly straight at the store, so it's important to make sure you've got a straight line when you get home!
Start by holding your fabric up, folded in half, with the selvage edges(that's that white line, remember?) matching up on the side. That should tell you if your top and bottom sides are straight or not. In the first photo, you can see that when the selvages don't match correctly, the fold of the fabric is billowy. In the second photo, I've matched the selvages correctly, so the fold lies flat. (you don't need a toddler with a book for this, but she thought it added a little something)
The fabric store cutter did fairly well, but it's clear here that I'll need to smooth out the edge a little:
Using a rotary mat and ruler, match up the top of the ruler with the selvaged edge to make sure that it will be an even cut. You can also use your pattern as a guide, if your fabric has one.
Notice that the ruler is flush with the selvage, and the row of hearts, but the edge that I cut off is uneven.
As you move the ruler down, use a couple of inches of the section you've already cut as a guide to keep the ruler straight.
Once you're all straightened up, you're ready to cut your piece! I always like to cut my pieces on the fold, if they're the same shape on both sides. For example, for this 5"x10" rectangle, I folded my fabric, and cut a 5"x5" square. When opened up, it's the right size!
(when using this method, fold over just the length you need from the edge, to avoid having a big hole in the middle of your fabric!)
If you don't have a rotary mat, cutter, and ruler, sell your car and buy one with the profit.
If that's not an option, you can try this!
Your fabric store should have something called quilter's chalk, or tailor's chalk. This comes in several different forms, but it usually only a few dollars. I've also heard that you can use white charcoal pencil in a pinch, which I had on hand!
It's very handy for marking out things before you cut them, which is a great way to ensure a straight line, when all you have are scissors to work with!
Cutting the same 5"x10" rectangle, use a tape measurer to measure 5" from the raw edge, and mark it at the fold:
Then, measure again, about 5" down. (my picture shows ten inches down, because I am silly, and forgot I was cutting on a fold), and mark a straight line along the tape measurer. Then use your tape measurer to draw a straight line between the two marks. Then cut along your lines!
If you make a mistake, you can use a scrap of batting to brush it lightly, "erasing" the chalk.