It took 20 years and much thought, discussion and negotiation to come to agreement on the decision to use 20 °C as the standard reference temperature for dimensional measurements. Doiron in his retrospective on this subject (page 1) presents original documentation about the role of the U.S. Bureau of Standards in this decision. The cover illustration implies that there were many choices all with good reasons for a standard reference temperature. Cover arranged by C. Carey.