Interfacing is a fabric which is used to make certain parts of a garment more stable. It is used as an additional layer which is applied to the inside of garments, such as collars, cuffs, waistbands and pockets, helping to add firmness, shape, structure, and support to the clothes.

What type of interfacing should I use?

Generally you should NOT use a heavier weight interfacing than the fabric, as the interfacing will 'dominate' the garment and add an unnatural structure to it. So for medium weight fabrics, use medium weight interfacing. For medium weight knit fabrics, use medium weight knit interfacing.

Do you have to use interfacing when sewing?

Even if using a naturally crisp or heavy material, you will need interfacing in structural areas so that they are less limp than the rest of your garment. ... Made out of only cloth, it would be like a pocket. It would sag and distort when you put things in it. Interfacing is what gives a purse the ability to hold a shape.