Casting off (or binding off if you are in the States) is the way you finish a piece of knitting. Some pieces will not be cast off, but only where the stitches will be used to continue knitting of if you intend to graft two pieces of knitting together.
For example in some patterns the stitches remaining on the back and sleeves of a jumper when you have finished knitting them are used along with stitches picked up on the edges of the front to knit a neck band.
And while there are several methods of casting on, most knitters use just one method of casting off - one stitch at a time as you work your way along the final row but there are a few decorative methods that you might like to try as your skills improve.
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