I-Cord

Beginner Edging

A narrow knitted tube worked on two double-pointed needles, used for ties, edgings, and straps.

Using double-pointed needles, cast on 3-5 stitches — 3 for a very narrow cord, up to 5 for a thicker one.
Knit across the stitches as normal, but do not turn your work.
Slide the stitches back to the other end of the same needle, so the working yarn is now at the wrong end.
Pull the working yarn tightly across the back of the stitches and knit across again.
Repeat the sliding and knitting — always from the same side — until the cord reaches the desired length.
To finish, either bind off normally or, for i-cord edging worked along a piece, work the last stitch of every row together with a picked-up stitch from the edge you're trimming.

The tube forms because you never turn the work — pulling the yarn snugly across the back each time is what pulls the edges around into a round cord.
The first few rows look messy and loose; the tube shape only becomes obvious after 4-5 rounds, so don't judge it too early.
I-cord bind-off, working 2 stitches together with the last i-cord stitch at the end of every row, gives a neat, cord-like finished edge to any knitted piece, not just standalone cords.