Early tapes were wound flat on a storage reel and transferred to a "take-up reel" as they were recorded or played. After recording or playback, they were "rewound" so that they were stored only on the storage reel. This kind of system is usually called "reel to reel" today. It is still used today for some professional recording and playback, but for home use reels were mostly replaced with other kinds of tape in the 1970s. Cassettes are small cartridges containing two reels inside. Cassettes move a four-track tape in either of two directions, corresponding to their "side A" and "side B" labels. The reel on the left contains the unplayed or unrecorded tape, and the reel on the right contains the tape that has already passed the recording or playback head. When you turn the cassette over, the tape still moves from left to right, but this is really the opposite direction. The "side A" recording plays two tracks as stereo, and the "side B" recording plays the other two.