A. The weight of the interlining. Weight is important, because it influences the style and design of the garment. Thick interlinings weigh down the garment and make it appear bulky, while thin pieces of fabric cause a garment to be light, but also less solid.
B. The stiffness of the interlining. Stiffness is not always desirable. An interlining that is too stiff will make a garment uncomfortable to wear, while an interlining that is too soft will not offer sufficient protection against cold or heat.
C. The color and appearance of the interlining. This is important for aesthetic reasons, since you don't want an ugly piece of cloth to show through your nice garment.
D. The size and shape of the interlining (i.e., whether it has straight or curved edges). Interlinings with straight edges are easier to work with than those with curved edges because they lie flatter against the inside of the garment and do not show through on the outside surface of the garment as much as pieces with curved edges do.
E. The thickness of the fabric used for the interlining (usually expressed as a number or a letter on the fabric's label). Thicker fabric feels warm and solid, while thinner fabric feels light, airy and less warm than thicker fabric does (see section Interlining Fabric Thickness).
F. The fabric used for the interlining must be easily washable, since it will probably need to be cleaned regularly during use (for example, if you spill something on yourself or get your clothes dirty). Also, you should never use wool fabrics as thermal linings in winter garments because they are too difficult to clean properly (they need special care if you want them to last long). If you accidentally spill something on a woolen piece of clothing, you cannot simply throw it into a washing machine; instead you have to hand wash it in lukewarm water and use a special soap for wool (called "wool soap").
G. It should be possible to obtain the interlining easily from your local sewing-supply store or from mail order companies that sell fabrics by mail order only (see chapter Faxing Sewing Patterns: Using Pattern Grids). This makes selection easier because you can see what is available in all different weights, types and colors before having to leave your home or office! You can even order different types of fabrics at one time, which saves you both time and money!
H. You should select fabrics that are mostly made from natural fibers such as cotton or wool, because they are more comfortable than synthetic fibers such as polyester or nylon (see section Interfacing Fabric Content). Synthetic fibers may feel nice at first when they are warm from storage or when they have just arrived at room temperature after being stored in a freezer (for example), but they absorb moisture easily and feel sticky when wet—which can be quite unpleasant! Also synthetic fibers usually cannot be washed in a regular household washing machine unless they have been specially treated beforehand so that they can withstand high temperatures during washing. It is therefore best if you avoid these fabrics whenever possible!
The thickness of an interlining must be selected carefully depending on its intended use in everyday life: A thinner lining feels more comfortable under ordinary day-to-day circumstances; but when used for skiing or riding on a motorcycle for long distances, a thicker lining is much more comfortable because it does not rub against your body nearly as much as a thinner lining does during these activities!