Crabtree (July Journal, page 525) has suggested a simple method for estimating the transition position on an aerofoil, based on the use of a single curve of momentum thickness Reynolds number (Rδ2) against a pressure gradient parameter at transition, mr. This curve makes some allowance for the effects of pressure gradient and laminar separation on the transition “point”, neglecting the effects of roughness, free-stream turbulence, and so on, but is considered “adequate for most practical purposes”. However, transition is a complex phenomenon, sensitive to a great variety of factors, so that Crabtree's method should be accepted with some reservations.