Basically, a lining is a replica of a garment that is attached to the inside of the garment at various points. It is an inner garment made of thin, silky fabric that is sewn on the inside of the fashion garment.
The following are excellent reasons why you'd line a garment.
It is used to cover internal constructions details.
Lining makes garments hang better, fit smoothly and hang more comfortably.
Lining a garment makes it easier to put on and take off.
Lining is often a hallmark of quality clothing.
A lining will conceal the raw edges of a garment and prevent fraying.
Lining provides a beautiful finish to armholes and neck edges.
It provides warmth.
It replaces the need for a slip.
Silk-type fabrics are often used for the lining fabric, however, there are many choices available. A general rule is that the lining fabric should never be heavier than the garment fabric. It is also wise to choose an anti-static fabric to avoid static cling in the finished project.
Typically the lining is cut the same dimensions as the fashion fabric, though if you want prevent some of the stress on the outer fabric, the lengthwise seams of the lining can be sewn just slightly wider than those of the fashion fabric. When wearing, the stress on the fabric will then be on the lining, not on the fashion fabric. Also, you'll want to shorten the hem of the lining about 1/2" to 1" shorter than the fashion fabric.
Some fabric choices for linings are:
Silk Crêpe de Chine - softly textured, available in a variety weights, provides excellent drape, does not fray.
Acetate Satin - beautiful drape and sheen, easy to take on and off, no static
Polyester Habotai (also know as China silk) - economical, durable, drapes well, wrinkle and stain resistant.
Though linings are often the same or similar color tones to the fashion fabric, you can also choose more bold colors or attractive prints.
There is a difference between lining and underlining. Underlining is a second layer of fabric cut from the pattern and sewn as one piece with the fashion fabric. It is used to add weight, firmness and stability to the fashion fabric.
As an alternative to lining, underlining backs the fashion fabric and is attached by hand or machine basting wrong side to wrong side of each garment piece before sewing the garment together. Then the underlining and the fashion fabric are treated as one piece.
This method does not conceal the raw edges, which must still be finished.
It does, however, reduce clinging, stretching and wrinkling.
It provides firmness to the outer garment and can reduce the buildup of static electricity in some fabrics.
Underlinings may also be referred to as Mounting.
Some fabric choices for underlinings are:
Cotton Voile - sheer and thin, provides stability for light to medium weight silk, absorbent
Rayon Challis - soft and light, adds depth but not bulk to garments
Cotton Flannel - provides warmth and absorbency, lightly napped surface
And then there's interlining. This is a term used for a layer that sits between the fashion fabric and the lining fabric. Batting or wool would be a good example for interlining.