If you draw an aerofoil, you can draw circles from the leading to traling edge which get bigger or small depending on the shape of the wing (just like the circle which determines the leading edge 'radius'). The top and bottom of the circles are just tangential to the upper and lower wing surfaces respectively, and so the centre of the circle is half way between the upper and lower surface. A line joining the centres of these circles will be curved for a cambered aerofoil, but straight for a symmetrical one.