A flat seat is easy to build and slightly wrong to lie on. This one is built to a curve, and the curve is set before a single spring goes in.
The problem with a flat day bed is that the human body is not flat. Lie down and the hips sink, the thighs stay high, and the heels want to be supported above the knees. On a flat platform the body finds its own position - usually by sliding gradually towards one end.
So the seat platform on this day bed is built to a profile. A template is cut to the reclining line first, and the springs are then set at varying heights along the frame to follow it: deeper through the hip zone, firmer and higher under the thighs, rising again towards the foot. Each spring is tied to its neighbours at the height its position calls for, not at a common level.
The buttoning is set out on that finished contour, not on a flat drawing, so the diamonds stay even where the surface curves.
The result is a piece you do not have to adjust yourself on. It holds you where it puts you.