Interfacing is a textile used on the unseen or "wrong" side of fabrics to make an area of a garment more rigid. It is an additional layer applied to the inside of garments or other sewing projects, in certain areas only, to add firmness, shape, structure, and support to areas such as collars, cuffs, waistbands, and pockets; and to stabilize areas such as shoulder seams or necklines, which might otherwise hang limply. Interfacing can be used to stiffen or add body to fabric, such as the interfacing. Interfacing has three main types: woven, non-woven, and knit, each designed to behave differently. Interfacings come in a variety of weights and stiffnesses to suit different purposes, and they are sold at fabric stores by the yard or meter from bolts, similar to cutting fabric. Sewing patterns specify if interfacing is needed, the weight of interfacing that is required, and the amount. Most modern interfacings have heat-activated adhesive on one or both sides, and they are affixed to a garment piece using heat and moderate pressure, from a hand iron, for example. This type of interfacing is known as "fusible" interfacing. Non-fusible interfacings do not have adhesive and must be sewn by hand or machine. The weight of interfacing to buy depends on the project, and it is worth doing a test using a scrap piece of fabric and interfacing before applying it to the main fabric.