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When you are learning how to knit, you will learn one of the more general purpose knitting cast on methods. And you will probably stick with that methods for most of your knitting career, but there are many different methods that you could use.
Some methods are general purpose and can be used for most knitting projects, but others are for a specific purpose (like a provisional cast on) that you will only use when your project has the need of them. In all probability, you are not even going to be aware of the special purpose methods until you attempt a pattern that uses them!
The Knitting Cast on Method I use most often:
Cable Cast On
I use this cast on method for most of the projects I work on. It is reasonably easy to do and it creates a firm, even cast on edge with a little bit of stretch. When I am working on something that starts with rib, I cast on knit and purl-wise to match the rib pattern, this makes the cast on edge almost disappear or at least not be as prominent.
For more information (step-by-step instructions) about using this method of casting on your knitting, check out this page: Casting Knitting On.
Thumb Method of Casting On
This is probably one of the easiest methods of casting on that you will come across. It involves working with a single needle in your left hand, forming stitches by wrapping the yarn around your right thumb and then transferring those stitches onto the needle.
Easy to do once you get your hands to work the way you want them to (can take a bit of practice until you are comfortable with the way you use your two hands) and it is quick. However it is not one of my preferred methods as the cast on edge can be a little unstable and the tension between stitches can vary making it less uniform than a cast on edge using the cable method.
Long Tail Cast On Method
The long tail method of casting on also only uses one needle, but it is held in your right hand rather than the left. Your slip knot is placed so that you have a very long tail (a little longer than your cast on edge will be) so that when it is on the needle you have two long tails under the stitch on the needle.
Basically what this cast on method involves is wrapping one of the tails around your left thumb and then knitting into the front of the loop with the other tail. As you form each stitch you have one tail at the back of the stitch and one at the front.
Provisional Cast On Method
A provisional cast on is usually done using a piece of contrasting yarn with the intention of removing the cast on edge later. You would use this type of cast on edge where you want to easily gather the bottom edge of your knitting or where you want to pick up the stitches and knit down.
I use a crochet hook to make a chain with a few more stitches than I need to create a provisional cast on. You can find out more about how I do it on this page.