Estimates of the effects of vorticity, baroclinicity, and rate of strain on the minimum and maximum propagation frequencies of gravity waves are given using a general dispersion relation for internal waves in the atmosphere and ocean. Rotating the coordinate axes to eliminate from the dispersion relation cross terms in the wavenumber makes it much easier to determine the minimum and maximum frequencies for which gravity waves can propagate. Corrections to the minimum frequency from breakdown of the WKB approximation are included.