Learn to Knit: Jeny's Stretchy Cast-On Tutorial
If you want a super stretchy cast on for the edge of your work this is a good one to start with.
It's frequently used for the top of socks and I've also used it for the edge of my Spritz Stripes sweater.
See it in action:
Photo Tutorial
1. Create slip knot and place on needle.
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2. With left thumb lift working yarn from back, twist, and sit backwards loop on needle placing index finger inside.
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3. Use thumb to bring yarn from front to back over needle and use index finger to lift first loop over yarn just put on needle.
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4. Pull yarn to tighten.
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Take care to keep stitches close together as it will be difficult to tighten up the stitches otherwise.
Read more about knitting cast on methods.