ISO 1

ISO 1 is an international standard set by the International Organization for Standardization that specifies the standard [reference temperature] for geometrical product specification and verification. The temperature is fixed at 20 degrees Celsius, which exactly equals both 293.15 kelvin and 68 degrees Fahrenheit.
Due to thermal expansion, precision length measurements need to be made at a defined temperature. ISO 1 helps in comparing measurements by defining such a reference temperature. The reference temperature of 20 °C was adopted by the CIPM on 15 April 1931, and this temperature was used in ISO recommendation number 1 in 1951. It soon replaced worldwide other reference temperatures for length measurements that manufacturers of precision equipment had used, including 0 °C, 62 °F, and 25 °C. Among the reasons for choosing 20 °C was that this was a comfortable and practical workshop temperature and that it resulted in an integer value on both the Celsius and Fahrenheit scales.
It was the first ISO standard, issued originally as ISO/R 1, an ISO Recommendation.