Michael of Luminous Landscape fame claims ownership of the Expose To The Right (ETTR) school of exposure having first proposed it in 2003. His latest version can be found HERE. It is based on the difference between film and sensor. In film the 10 zones (except 1 and 10) can carry equal amounts of tonal information, (Zones 1 and 10 being pure black and white carry no tonal info). In a digital sensor the zones carry increasing amounts (bits) of tonal values and Michael has the maths to prove it. Note this does not recommend clipping and it only applies to RAW images where no exposure curve is applied in camera.