IEC 61300-3-51:2014 is intended to describe the procedure required to measure the pin gauge withdrawal force for rectangular ferrule multi-fibre connectors. This measurement can be used as an alternative to the measurement of the pin gauge insertion force, both of which evaluate the fit of the pin gauge(s) into the guide pin hole(s) of the female-type connector. The experimental verification of interchangeability between them is shown in Annex A. For the male-type connector, IEC 61300-3-49 is applied to evaluate how securely the guide pins are retained in the guide holes. Keywords: pin gauge withdrawal force, pin gauge insertion force