The standard magnetic tape system (called tape type 3) used half-inch-wide (12.7 mm) magnetic tape with ten tracks at a density of 300 bits per inch. Four of the tracks held data, four more tracks held the inverse of the data and there was a parity bit for both groups of four. This allowed single-bit errors to be corrected and double-bit errors to be detected. Up to eight Ampex TM4 tape decks, running at 75 inches per second for a throughput of 22,500 digits per second, could be connected. Spools could hold up to of tape and were of the three-prong design common at the time for professional audio recorders rather than the later industry standard expanding-hub design.