A padded jacket keeps you warm because of the air it holds, not because of the fibres themselves. Cotton, silk floss or down are all just structures for stopping air from circulating: the more still air trapped per centimetre of thickness, the slower heat leaves the body. That part is simple. The trouble is that a loose layer of fibres has no loyalty. It moves.
Wear an unquilted padded layer for a week and the filling settles. Gravity pulls it toward the hem, a shoulder strap presses it out of the upper back, and the bend of an elbow pushes it toward the seams. Wash it once and the fibres mat into lumps that no amount of shaking separates again. A garment that began as an even blanket ends up as a warm hem and a cold chest.
Grid as containment
Quilting divides one large pocket into many small ones. Each line of stitching pins the outer cloth to the inner cloth, so the filling in a given cell cannot cross into the next one. Migration is limited to the width of a single cell. Make the cells smaller and the filling stays put more reliably; make them larger and you get more loft in the middle of each cell, where the two layers are free to billow apart.
That trade-off is why the spacing is a decision rather than a pattern. Every stitch line compresses the two cloths together and squeezes the filling flat along that line, leaving a narrow cold bridge where there is almost no trapped air. More rows mean better containment and more cold bridges. Fewer rows mean better insulation on day one and a lopsided garment by the end of the season. Old workwear tends to use close, plain rows; a decorative quilt often uses wider ones, and pays for it later.
The thread itself matters less than the fact that it goes all the way through. A line sewn only into the top layer would hold the surface still while the filling went on moving underneath. Quilting is a through-stitch: it joins face, filling and lining into one sheet that can be lifted, folded and washed as a single piece of cloth.
Why the Chinese padded jacket is built in panels
In the traditional cotton padded jacket the quilting often happens before the garment is completed. Flat panels are padded and stitched on a table, then cut and joined, so the seam allowances become additional fixed edges. This is a different order of operations from cutting a shaped jacket first and trying to pad it afterwards — a curved, three-dimensional shell is hard to stitch evenly on a machine, while a flat panel is not. The construction follows from what the needle can reach.
It also follows from washing. In many versions the face and lining can be separated from the filling, so the cloth is laundered and the padding is aired rather than soaked. Cotton wadding that is repeatedly wetted and dried loses loft in a way that no amount of quilting can reverse. Keeping the filling out of the wash is a maintenance decision that was designed into the garment from the start.
Compare wool felt, which needs none of this. Felt fibres lock into each other under warmth, moisture and pressure, so the sheet holds itself together and cannot migrate. Loose wadding has no such self-locking, and thread has to supply the structure. The same distinction is at work in garments cut from a single rectangle of cloth, where the belt rather than the stitching does the shaping.
Quilting also changes where the garment wears out. Friction concentrates at collars, cuffs and underarms, and a stitch line running through one of those zones gives abrasion a ready-made ridge to work on. Well-made quilting therefore avoids putting a row exactly across a high-friction fold, or reinforces that row with a facing. The grid has to respect the map of wear, not just the shape of the panel.
So the lines on the outside are not decoration applied to a warm garment. They are the reason it stays warm in the same places year after year — a small, visible account of how much a loose material can be trusted to stay where it was put.