Whitworth and BSF spanner markings refer to the bolt diameter, rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion can arise because each Whitworth hexagon was originally one size larger than that of the corresponding BSF fastener. This leads to instances where for example, a spanner marked " BSF" is the same size as one marked " W". In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same.