Compact Cassettes contain two miniature spools

Compact Cassettes contain two miniature spools, between which the magnetically coated, polyester-type plastic film (magnetic tape) is passed and wound[12]—essentially miniaturizing reel-to-reel audio tape and enclosing it, with its reels, in a small case (cartridge)—hence "cassette".[13] These spools and their attendant parts are held inside a protective plastic shell which is 4 by 2.5 by 0.5 inches (10.2 cm × 6.35 cm × 1.27 cm) at its largest dimensions. The tape itself is commonly referred to as "eighth-inch" tape, supposedly 1⁄8 inch (0.125 in; 3.17 mm) wide, but actually slightly larger, at 0.15 inches (3.81 mm).[14] Two stereo pairs of tracks (four total) or two monaural audio tracks are available on the tape; one stereo pair or one monophonic track is played or recorded when the tape is moving in one direction and the second (pair) when moving in the other direction. This reversal is achieved either by manually flipping the cassette when the tape comes to an end, or by the reversal of tape movement, known as "auto-reverse", when the mechanism detects that the tape has ended