Quad 8

Quadraphonic (Quad 8) Sound: Quadraphonic audio (aka Surround Sound) adds rear channels (aka Surround Channels) to Stereo Audio.
Introduced in 1970, Quad 8 Tape Cartridges resemble Stereo 8 Cartridges.
Quad 8 tapes allocate tape tracks differently, combining tracks 1, 3, 5, and 7 to Program 1 and combining tracks 2, 4, 6, and 8 into Program 2.
Quad 8 Cartridges contain a small vertical notch in the top left corner, so the Quad 8 player can automatically set up the proper program/track configuration.