A dress that holds itself up
A structured bodice doesn't grip the body — it stands away from it, a small load-bearing shell that happens to be worn. The boning is what makes that possible.
Steel bones, either flat or spiral-wound, run vertically through channels stitched into the lining. Flat steel holds a clean, unbending line — useful at the front and back where the silhouette needs to be precise. Spiral steel flexes with movement, which is why it runs at the sides where the body bends. Each bone is tipped with a metal or nylon cap to stop it working through the fabric over the course of a day. The channels themselves are cut on the grain, sewn tight enough that the bone cannot shift but not so tight that it distorts the cloth above it.
The weight problem is simple in principle and awkward in practice. A full skirt — metres of silk, layers of net, a horsehair-stiffened hem — can be considerable. If that weight hangs from a waist seam alone, it pulls the bodice down over the course of an evening. The solution is to transfer the load inward, to the boning, which carries it against the ribcage rather than letting it drag at the fabric. A waist stay, a ribbon or petersham tape tacked to the boning at the natural waist and tied inside, takes the skirt's weight entirely off the outer seams. The outer cloth floats; the structure carries.
Boning also determines where the eye goes. The number of bones, their spacing and the curve cut into each one collectively shape the silhouette before a single body has been fitted inside. A maker adjusts the curve on a bone with long-nosed pliers, bending steel by a few millimetres until the resulting arc matches the toile's corrections. That adjustment — modest, slow, irreversible if overdone — is what makes a generic bodice fit a particular person.
The wearer feels almost none of this. That invisibility is the whole point.