While Burrows's intuitive and elegant ‘Delta’ measure for authorship attribution has proven to be extremely useful for authorship attribution, a theoretical understanding of its operation has remained somewhat obscure. In this article, I address this issue by introducing a geometric interpretation of Delta, which further allows us to interpret Delta as a probabilistic ranking principle. This interpretation gives us a better understanding of the method's fundamental assumptions and potential limitations, as well as leading to several well-founded variations and extensions.