Underlining

Underlining is easy to confuse with lining, but they do different jobs. A lining is a separate, complete inner garment sewn in afterward, mostly hidden and only lightly attached at the edges. Underlining is basted directly to the wrong side of each individual fashion-fabric piece before construction even starts, so the two layers behave as one from that point forward — every dart, seam, and edge is sewn through both layers together.

The point of it is support without changing the outer fabric's own look or texture. A silk that's too sheer or too flimsy to hold its shape alone gets underlined with a firm, sheer support fabric (silk organza is the classic choice) so it drapes and holds structure without adding visible bulk or a different surface texture the way a heavier fashion fabric substitution would. It's also how a lot of tailored garments get subtle internal shaping without any of that shaping showing from the outside.

I baste underlining to each piece by hand, close to what will become the seamline, before doing anything else to that piece — treat it as step one, not an afterthought layered on partway through construction.