"[M]any people are probably not aware of the extent to which systematic differences exist within languages. Internal variation referrers to the fact that within a single language, there are different ways of expressing the same meaning. This property is inherent to all human languages and to all speakers of a language."
Vedrana Mihalicek and Christina Wilson (Eds.) 2011. Language Files: Materials for an Introduction to Language and Linguistics. Columbus: The Ohio State University Press, 408.