Webster's 1828 dictionary featured only -or and is generally given much of the credit for the adoption of this form in the US. By contrast, Dr Johnson's 1755 dictionary used the -our spelling for all words still so spelled in Britain, as well as for emperour, errour, governour, horrour, tenour, terrour, and tremour, where the u has since been dropped. Johnson, unlike Webster, was not an advocate of spelling reform, but selected the version best-derived, as he saw it, from among the variations in his sources: he favoured French over Latin spellings because, as he put it, "the French generally supplied us."[24]
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American and British English spelling differences - Wikipedia, the free encyclopedia