From The Art and Popular Culture Encyclopedia
To hem a piece of cloth (in sewing), a garment worker folds up a cut edge, folds it up again, and then sews it down. The process of hemming thus completely encloses the cut edge in cloth, so that it cannot ravel.
A hem is also the edge of cloth treated in this manner.
The hem may be sewn down with a line of invisible hem-stitch or blind-stitch or sewn down by a sewing machine, usually leaving a visible line of sewing. Modern sewing machines designed for home use can make many decorative or functional stitches, so the number of possible hem treatments is large. These home-use machines can also sew a reasonable facsimile of a hem-stitch, though the stitches will usually be larger and more visible.
Clothing factories and professional tailors use a blind hemmer, or hemming machine, which sews an invisible stitch quickly and accurately. A blind hemmer sews a chain stitch, using a bent needle, which can be set precisely enough to actually sew through one and a half thicknesses of the hemmed fabric.
Most haute couture hems are sewn by hand.
Heavy material with deep hems may be hemmed with what is called a dress-maker's hem — an extra line of loose running stitch is added in the middle of the hem, so that all the weight of the cloth does not hang from one line of stitching.
The term hem is also extended to other cloth treatments that prevent raveling. Hems can be serged (see serger), hand rolled and then sewn down with tiny stitches (still seen as a high-class finish to handkerchiefs), pinked with pinking shears, piped, covered with binding (this is known as a Hong Kong finish), or made with many other inventive treatments. there are many types of hemming.
Hem repair tape is available as an alternative solution to sewing a broken hem. To effect a fix, the hem repair tape is laid around the inside of the hem. It is then ironed with a hot iron. The heat causes the tape to bond the two surfaces together.