▁Width ▁across ▁fl ats ▁is ▁the ▁distance ▁between ▁two ▁parallel ▁surfaces ▁on ▁the ▁head ▁of ▁a ▁sc rew ▁or ▁b olt , ▁or ▁a ▁nut , ▁mostly ▁for ▁tor que ▁transmission ▁by ▁positive ▁lock ing . ▁ ▁The ▁term ▁width ▁across ▁fl ats ▁( AF ) ▁is ▁used ▁for ▁the ▁following ▁forms : ▁▁ 2 - socket ▁= ▁round ▁material ▁with ▁two ▁surfaces ▁ 4 - socket ▁= ▁profile ▁square ▁section ▁ 6 - socket , ▁ 8 - square ▁= ▁regular ▁pol yg ons ▁ ▁Sp anner ▁The ▁width ▁across ▁fl ats ▁indicates ▁the ▁" size " ▁of ▁the ▁sp anner . ▁It ▁is ▁im print ed ▁on ▁the ▁sp ann ers ▁in ▁millimeter ▁( mm ) ▁values . ▁Old er ▁English ▁and ▁current ▁American ▁sp ann ers ▁( w rench es ) ▁have ▁in ch ▁sizes ▁that ▁are ▁im print ed ▁in ▁intermediate ▁sizes ▁in ▁fra ctions . ▁ ▁The ▁two ▁systems ▁are ▁in ▁general ▁not ▁compatible , ▁which ▁can ▁result ▁in ▁round ing ▁of ▁n uts ▁and ▁bol ts ▁( i . e . ▁using ▁a ▁ 1 3   mm ▁sp anner ▁in ▁place ▁of ▁a ▁). ▁There ▁are ▁some ▁exceptions ▁with ▁a ▁few ▁sizes ▁being ▁close ▁enough ▁in ▁sizes ▁to ▁inter change . ▁This ▁includes ▁ 1 9   mm ▁and ▁  in ▁( 1 9 . 0 5   mm ), ▁which ▁are ▁inter change able ▁for ▁most ▁purposes . ▁S izes ▁that ▁may ▁inter change , ▁depending ▁on ▁the ▁precision ▁needed , ▁includes ▁ 2  