A comparison is made between several shear layer refraction theories to determine their relationship to one another and to determine which parameters are important for an open jet wind tunnel shear layer correction. For sound transmission through a parallel sheared flow, the shear layer thickness is found to be unimportant at Mach numbers typical of open jet tunnels. The effect of reflected waves, although more significant, can usually be ignored, allowing a correction which is independent of source type and frequency. The shear layer shape (plane or cylindrical) can be important and the correction corresponding to the actual shear layer shape should be used. The numerical solutions of the Lilley equation for the limiting cases of a thick and a thin shear layer are found to agree with the algebraic expressions given for these limiting cases.