See https://en.wikipedia.org/wiki/Dolby_Stereo https://en.wikipedia.org/wiki/Dolby_Stereo > The optical soundtrack on a Dolby Stereo encoded 35 mm film carries

See https://en.wikipedia.org/wiki/Dolby_Stereo https://en.wikipedia.org/wiki/Dolby_Stereo > The optical soundtrack on a Dolby Stereo encoded 35 mm film carries not only left and right tracks for stereophonic sound, but also—through a matrix decoding system (Dolby Motion Picture matrix or Dolby MP[1]) similar to that developed for "quadraphonic" or "quad" sound in the 1970s—a third center channel, and a fourth surround channel for speakers on the sides and rear of the theater for ambient sound and special effects. This yielded a total of four sound channels, as in the 4-track magnetic system, in the track space formerly allocated for one mono optical channel. Dolby also incorporated its A-Type noise reduction into the Dolby Stereo system.