Smooth stretch jersey with molded foam cups built into the interior lining — the structure comes from the cups, not boning, so it moves with the body. Hand wash cold to protect the cup shaping, and avoid folding it in a way that creases the foam, which can leave a crease line that doesn't fully release. Avoid the tumble dryer, which can warp the molded foam permanently, and lay flat to dry rather than hanging, which pulls at the cup seams under the fabric's own weight.