This International Standard specifies the size pairing of double-ended wrenches. It applies not only to flat wrenches for nuts but also to all wrenches with two fixed ends for screws and nuts, such as socket wrenches. This International Standard specifies preferred pairings of widths across flats in accordance with ISO 272 (see Table 1). It also gives in Table 2 other pairings still widely used but non-preferred.