A method for calculation of the integral reflection coefficient of crystals of interrnediate perfection is introduced. This method can greatly reduce experimental effort for the selection and calibration of crystals, It also serves as a conceptual framework for studies of mosaic block structure and of crystal modification. Good agreement between calculated and experimental values of the integral reflection coefficient is shown for, (a) LiF crystals of two degrees of perfection, (b) elastically bent quartz, and (c) 001, 005, 006, and 007 diffraction from KAP. Zachariasen's division of crystals into two types is extended. It is concluded that the integral reflection coefficients for 200 LiF cannot be raised to the ideally imperfect limiting values.