Some SLR cameras has semi-transparent mirrors to allow some light to pass through them to autofocus sensors or light metering cells under the mirror. If you have such a camera, you need a circular polarizer because a linear polarizer causes inaccurate meter readings and interferes with autofocus.

All AF 35mm SLRs are like this. Several manual-focus cameras are too, like the Nikon F3, Olympus OM-4/4T, OM-3/3Ti, OM-PC, and OM-2sp