This International Standard specifies file structures for data interchange on magnetic tape cassettes. To provide for the range of sophistication in equipment and applications, three systems are specified : a) the basic system, employing only hardware-defined separators to structure the files; b) the compact system, employing special data blocks with information content (labels), which are capable of being recorded using only numeric equipment; c) the extended system, employing the magnetic tape labelling system specified in ISO 1001 together with new labels, to define a more comprehensive labelling system. This International Standard is not limited to the 3,81 mm magnetic tape cassette described in ISO 3407 but could also be applied to higher capacity cassettes or cartridges.