Form of thread defined. Tables: basic sizes, limits, tolerances for 'class' bolts, nuts; BS Whitworth range, $9B in to 6 in; BS Fine range, $9C in to 4$9C in (limits and tolerances up to 3 in only). Also a Selected Thread series more suitable than BS Pipe parallel series for extra fine pitch applications. Appendices: limits, tolerances for B.S.W. and B.S.F. truncated bolts and nuts; bases of tolerances and allowances, formulae; effects of pitch and angle errors, acceptability of error combination; form of Whitworth crests ; limits and tolerances for (non-preferred) 'close class' nuts.