Ahhh, finally done with your latest knitting project. Now you can't wait to finish so, in a final flurry, you bind off all your stitches and...oh no. The sweater won't fit over your head or the bound off edge of the blanket is narrower than the cast on edge.
What have you done? You bound off too tightly.
I've done it myself. You might not notice if it's a scarf because a scarf is narrow. The bound off edge does not have as much "give" as the rest of the knitting. That's why it's difficult to get the neckline of that sweater to stretch enough to fit over your head.
1. Bind Off Loosely
Always, always, always bind off loosely. This includes the stitches that you are knitting or purling during the process as well as when you pass a stitch over and off. Don't tug, pull, or yank the yarn as you work each stitch. I know that it seems so loose that it's tempting. But don't. If you find you are binding off too tightly and can't manage to do it more loosely, use needles one or two sizes larger than the size you used to knit the piece.
Binding off, sometimes called casting off, actually creates a final row of fabric, so what stitches you work as you bind off does make a difference. You can simply knit across as you bind off as many people do; but upon close inspection you'll see the difference in the details.