Abstract: A mat for a person to lie on has an air-permeable top layer (1), a bottom layer (2), and a middle layer (3) which includes an open three-dimensional knitted structure with mesh openings (5) in the top layer, which is of an open-mesh, air-permeable fabric. The structure is formed of evenly-spaced, substantially rigid monofilaments (4, 4') between the top and bottom layers. The middle layer is substantially incompressible a person's weight. In the middle layer air occupies at least five times the volume of the monofilaments and because of the open knitted structure air can flow through the middle layer and the top layer. The filaments preferably include both perpendicular monofilaments (4) and cross-over monofilaments (4') which can brace diagonally across the mesh openings (5). The monofilaments are preferably woven into the top and/or bottom layers.