On the rationale for recent updates to the SI Brochure concerning angles and quantities with the unit one

Abstract This paper documents the recent evolution of the International System of Units (SI) Brochure, the official document describing the SI, the only globally agreed system of units. In particular, the changes implemented in the most recent version of the 9th edition of the SI Brochure, version 3.01, published in August 2024 are described, along with the rationale for these updates. It is explained that these updates originated from work done by the International Committee for Weights and Measures’ Consultative Committee for Units and have concentrated on the treatment of angle and quantities with the unit one. The aim of the changes in these specific areas has been to provide clarification, harmonise language, improve understanding, acknowledge nuance and reflect new consensus in the understanding of these quantities and their application. By outlining the process and rationale behind these updates, it is hoped that users of the SI Brochure are provided with improved transparency and a better understanding of the ongoing development of this essential document.