Dolby Digital can, I believe, include any number of tracks less than or equal to 5.1 -- so you can have Dolby 2.0 tracks (stereo, left and right channels only), Dolby 1.0 (mono), Dolby 4.0 (left, right, center, and mono surround), Dolby 5.0 (all channels but no LFE channel), or whatever.

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