The VCR format used large square cassettes with 2 co-axial reels, one on top of the other, containing half inch (12.7 mm) wide chrome dioxide magnetic tape. Three playing times were available: 30, 45 and 60 minutes. The 60-minute videocassettes proved very unreliable, suffering numerous snags and breakages due to the very thin 17μm video tape. Tapes of 45 minutes or less contained 20 μm thickness tape. The mechanically complicated recorders themselves also proved somewhat unreliable. One particularly common failing occurred should tape slack develop within the cassette; the tape from the top (takeup) spool may droop into the path of the bottom (supply) spool and become entangled in it if rewind was selected. The cassette would then completely jam and require dismantling to clear the problem, and the tape would then be creased and damaged.