The provisional cast on is, as the name implies, a temporary cast on row. It is done with waste yarn so that you can take it out later and have "live" stitches in your working yarn. Waste yarn should generally be a contrast-color yarn (so that it’s easy to locate) and in a smooth, non-grippy fiber (to make it easier to rip out later). This technique is used in projects like infinity scarves because you can join the ends of your work so that it looks seamless. This invisible seaming (known as grafting) is achieved by doing a kitchener stitch with the live stitches that you will pick up from your cast on row. The provisional cast on is also used when you’ll be picking up the stitches in order to work the piece in the other direction (seen sometimes in patterns that feature lace designs, for example). Only use this cast on if directed by your pattern or if you’ll be grafting or picking up the stitches.
There are a few different ways to do a provisional cast on but we are going to do the version that utilizes a crochet hook. Let's walk through how you work this technique...
1. With a crochet hook make five chain stitches with your waste yarn.
2. With your left had hold your chain stitches and a knitting needle. Bring the yarn behind the knitting needle and wrap it around the index finger of your left hand (the way you would if you were doing continental knitting). Your crochet hook should still be in the last loop of your chain stitches.
3. Reach your crochet hook over your knitting needle to grab the yarn.
4. Pull the yarn through your loop. This is essentially the same motion you were making when doing the chain stitches.
5. Move your yarn behind the knitting needle again. Repeats steps 3 and 4 until you've made the desired amount of stitches.
6. Make five more chain stitches. Cut the yarn, tie a knot at the end, and pull the knot through the last chain stitch. This is now your cast-on row. From here you will attach your working yarn and knit as you normally would.
Once you've finished knitting your piece you will be ready to take out your waste yarn and pick up the "live" stitches. Here is how it works:
1. Find the end of your cast on row where you knotted the yarn.
2. Pull out the chain stitches. Gently pull the waste yarn out of your first cast on stitch.
3. Slip a needle into the "live" stitch that has just been revealed.
4. Continue in this manner across the row. Discard the waste yarn.
Now that you have the "live" stitches on your needle you can perform the kitchener stitch in order to seamlessly graft them to another knitted piece.