In knitting, casting on is a family of techniques for adding new stitches that do not depend on earlier stitches, i.e., having an independent lower edge.
In principle, it is the opposite of binding off, but the techniques involved are generally unrelated.
The cast-on can also be decorated with various stitch patterns, especially picots.
The cast-on stitches can also be twisted clockwise or counterclockwise as they are added to the needle; this is commonly done for the single cast-on described below to give it a neater, more uniform look.
Casting on is sometimes done with doubled-up needles or a needle of larger size than for the main pattern; the extra bit of yarn in each stitch makes the edge less tight and gives it more flexibility.