Stretchy cloth is spread in stacks and cut into cups, wings, center pieces and straps. The cloth is usually nylon or polyester with a small share of spandex, a fiber that can stretch up to five times its own length and spring back.

A thin layer of foam is laid against the inner face of the cut cloth and the two are hot-pressed together. Heat makes the foam tacky, so it grips the fabric with little or no extra glue.

Two foam layers go into a heated mold with a cup-shaped cavity on each side and a slim forming rib sandwiched between them. The mold closes under heat and pressure, and when it opens the foam keeps its rounded shape. Pulling the rib out leaves an arched tunnel along the bottom of the cup.

Cutters follow the outline of each pressed cup and clip away the flash around it. What remains is a finished shell with a ready-made channel for the wire.

Thin strips of steel, or of nickel titanium, a shape memory alloy, are bent into a U and given a nylon coating at both ends. The soft tips stop the metal from poking through the cloth after months of wear and washing.

Machinists stitch the cups to the center piece and to the side wings, then run elastic along the bottom band and the edges. The elastic carries most of the lift, and the straps mostly hold things in place.

A wire is pushed into the channel under each cup, running from the center out to the armpit. The open end of the channel is then stitched or capped shut so the wire cannot work its way out.

A strip of bent wire hooks goes on one end of the band and a matching strip of eyes on the other. Adjustable straps are sewn on last, and each piece is checked against its size before packing.