The planform area of a wing is the area of a wing as if it were projected down onto the ground below it. It is represented as the variable S. It can be obtained from the mean chord c and wingspan b: S=b*c. It is a common reference area used in aircraft design to make dimensionless numbers. It is also part of a shortcut used to find the aspect ratio for wings with multiple cross sections and/or tapering, AR=b²/S. It differs from the surface area because surface area is the total area of the skin covering the wing in 3D, but the planform area is measured in two dimensions; the planform area is somewhat less than one half of the surface area.