d to the Whitworth 55° and very fine thread pitches.</p>
<p>(To simplify matters the term hexagon will be used in this paragraph to denote either bolt head or nut.) Whitworth (<a href="//en.wikipedia.org/wiki/Wrench" title="Wrench">spanner</a>) markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as in other standards. Confusion also arises because BSF hexagon sizes can be one size smaller than the corresponding Whitworth hexagon. This leads to instances where 