Final answer:
Millinery Interlining is the heavy, stiff fabric used to support the edge of a hem or sleeve in clothing and hats.
Step-by-step explanation:
The heavy, stiff fabric used as a foundation to support the edge of a hem or sleeve, especially in millinery, is called Millinery Interlining. This type of material is used to provide structure and maintain the shape of various clothing articles, including hats, which is a central aspect of millinery work.
Millinery Interlining is often made from materials such as buckram or other stiffened fabrics that can hold their form under the stress of wear and the natural elements.