The value of the yarn-on-yarn friction coefficient is an important parameter in analyses of the behaviour of fibre ropes and textiles. The standard method for calculating this friction coefficient for rope yarns relies on an analysis of the measured forces in a simple plait as a yarn is pulled over itself through the plait. Unfortunately, this analysis includes some ambiguities that have made their way into an ASTM standard and some worrying approximations. These issues are addressed here and then an improved analysis developed. Sample calculations indicate that differences of up to 40% in the calculated friction coefficient can be expected, with significant effects on any subsequent rope analysis.