ISO 691:2005规定了螺钉和螺母（或类似零件）的扳手和套筒开口公差，这些螺钉和螺母（或类似零件）具有公制宽度和尺寸。它给出了每个尺寸的通用公差，由其相对于标称值的两个偏差（最小和最大）定义。这些偏差是根据ISO 4759-1中规定的螺钉和螺母的平面宽度公差以及紧固工具的制造确定的。

ISO 691:2005 specifies tolerances on wrench and socket openings for screws and nuts (or similar parts) having metric width across flats dimensions. It gives, for each dimension, the tolerances for general use, defined by its two deviations, minimum and maximum, with respect to the nominal value. These deviations have been determined in relation to the width across flats tolerances for screws and nuts specified in ISO 4759-1 and with regard to the manufacture of tightening tools.