ESDU 74036 gives curves, derived from a correlation of experimental results obtained on a plate at zero incidence, for predicting the increment in drag for Mach numbers less than 3. The curves apply to a cavity with sharp edges, walls normal to the plate, flat bottom and no through flow; no information is available on rounding or chamfering the corners, but it should not be assumed to be beneficial. The depth/diameter ratio varied from 0.04 to 1.5 but a method of dealing with a cavity in which the ratio is less than 0.04 is explained. The data apply strictly in zero pressure gradient but guidance on their use where there is a pressure gradient is given.