IEC 61280-4-1: 2019 is applicable to the measurement of attenuation of installed optical fibre cabling plant using multimode optical fibre. This cabling plant can include multimode optical fibres, connectors, adapters, splices, and other passive devices. The cabling can be installed in a variety of environments including residential, commercial, industrial, and data centre premises, as well as outside plant environments. The test equipment used in this document has one single fibre connector interface or two single fibre connector interfaces. In this document, the optical fibres that are addressed include sub-categories A1-OM×, where × = 2, 3, 4 and 5 (50/125 m) and A1-OM1 (62,5/125 m) multimode optical fibres, as specified in IEC 60793-2-10. The attenuation measurements of the other multimode categories can be made using the approaches of this document, but the source conditions for the other categories have not been defined. This third edition cancels and replaces the second edition, published in 2009.