There was a fundamental change in our standards of length during the twentieth century, as the metal bars that had served for so long gave way to standards based on the wavelength, and then velocity, of light. This article describes those changes with particular emphasis on the standards-in-use of practitioners at the frontier of precision: spectroscopists, industrialists, geodesists, and astronomers. It reveals a multiplicity of de facto standards that supported their various innovations in practical electromagnetic measurement techniques. These de facto standards often came before, and guided the adoption of, the more visible new de jure standards that have been the subject of greater historical attention. I also offer a new explanation of the processes of change in our standards of length, based not simply on the search for better precision but more on concepts of mutual grounding and coherence.

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