Multiple sources abound in language, at all levels of linguistic analysis (phonology, syntax, semantics, etc.), and in a range of historical pursuits, including etymology and variationist investigations. From a methodological standpoint, moreover, recognizing multiple sources is often good historical linguistic practice (contrary to inclinations towards neat and elegant solutions that satisfy Occam&#8217;s Razor). That is, if we can identify multiple pressures on some part of a language system, it cannot always readily be excluded that some or even all might have played a role in shaping a particular development; if all of the factors represent reasonable pressures that speakers could have been aware of and influenced by, excluding any could simply be arbitrary. In this paper, accordingly, I survey the breadth of multiple sources in a variety of areas of language change, and advance one particular consequence that multiple sources can lead to, namely the hypothesis that recognizing multiple sources can be a basis for positing proto-language variation that is realized in variation within single languages and across related languages.