A small notch could stop a recording
On a standard compact cassette, the two small plastic tabs near the top corners marked whether each side could be recorded over. When a tab was intact, a cassette deck could treat that side as writable. Break it off, and the resulting opening signaled that the side should be protected.
The tabs did not alter the magnetic tape or erase anything. They worked as a physical cue for the deck: a small switch or sensor near the cassette compartment detected whether the opening was covered. If it found a missing tab, the deck disabled recording, helping prevent an accidental new recording from replacing what was already on the tape.
Protection without a lock
The safeguard was deliberately simple. A strip of tape over the opening could cover the sensor’s path and make the cassette appear recordable again. That meant the protection was a reminder built into the cassette’s shape, not an encryption system or an irreversible change. Anyone willing to cover the hole could restore the deck’s permission to record.
Each corner corresponded to one side, so a cassette could be protected on one side while remaining recordable on the other. That was useful for keeping a favorite album or an important recording intact without giving up the cassette’s reusable side entirely. The tiny tabs turned a choice about magnetic tape into something visible—and easy to reverse.
