Team flat sheet on top, and either fitted or flat underneath. I have an aircell blankie under the doona for winter, over the flat sheet on top. Comfy and flexible. The undersheet I really don't give a damn as both are functional, but flat sheets are MUCH easier to wash (and dry!) than fitted and in winter that does make a difference so flat sheet is easier to wrangle at this time of year. In summer when washing them is easy then fitted as the undersheet is preferable as cooler and less likely to get into a sweaty tangle.