I want to create a sharp edge between two smooth mesh surfaces. The upper surface has already been smoothed out nicely, merely using the Shading/UVs > Smooth Face function in the Tool Box (top left of picture), after selecting all its faces. However, when I do the same thing in the lower surface, the common edge between the two surfaces (= top edge of selected faces in picture) is also smoothed out (the picture is before that operation), which I don't want. I tried Shading/UVs => Smooth Edge=> Sharp, but that does not work. I don't want to use a modifier (because I want to control things intra object).
In Edit Mode, select the edges that you want to sharpen, and press V, then right click to exit transform, this will split the edges and create a crease edge. If you want to combine them later, you can simply select the vertices in Edit Mode with Ctrl+V and select (Remove Doubles).