From the Dolby FAQ:

Dolby Surround Pro-Logic emerged in home theater systems in the early 1990's.
It became the surround sound standard for Hi-Fi VHS and Laserdisc creating
dynamic 5.1, and is still the standard for today's analog TV broadcasts, since
the Dolby Surround Pro-Logic signal can be encoded in a stereo analog signal.
If you have an "older" Dolby Surround Pro-Logic receiver, you can still enjoy
movies from DVD-Video, since all DVD-Video players down-mixes the Dolby Digital
information to the Dolby Surround Pro-Logic format, and outputs the signal as a
stereo audio pair.

Before you start huffing about matrixing of channels, know that even the center
channel is a matrixed channel! The rear center is derived in the same manner.
My non EX discs still send SAE to the rear center. The rear center channel is
pretty important and is now SOP in most better theatres.

Want links on EX and 6.1 systems? Know why I don't have a 7.1 system?

RB