IBM 727

The tape had seven parallel tracks – six for data and one to maintain parity. Tapes with character data (BCD) were recorded in even parity. Binary tapes used odd parity. Reflective strips were glued several feet from the ends of the tape to serve as physical beginning and end of tape markers. Write protection is provided by a removable plastic ring in the back of the tape reel. Installing the ring enables writing, thus strictly speaking, it is a write enable ring, and removing it protects the tape from being written to.

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  An IBM 704 mainframe with IBM 727 tape drives on the left. (image courtesy of LLNL)
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  Half-inch tape reel with write enable rings
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  Model of an IBM 7272 tape drive