Free variation

In linguistics, free variation is the phenomenon of two (or more) sounds or forms appearing in the same environment without a change in meaning and without being considered incorrect by native speakers. Sociolinguists argue that describing such variation as "free" is very often a misnomer, since variation between linguistic forms is usually constrained probabilistically by a range of systematic social and linguistic factors, not unconstrained as the term "free variation" suggests. When phonemes are in free variation, speakers are sometimes strongly aware of the fact (especially if such variation is noticeable only across a dialectal or sociolectal divide). In the case of different realizations of the same phoneme, however, free variation is exceedingly common and, along with differing intonation patterns, variation in realization is the most important single feature in the characterization of regional accents.

Sources:

Meyerhoff, M. (2011). Introducing Sociolinguistics. London.

SIL International. (2003). Glossary of Linguistic Terms. Dallas (Texas).

Clark, J., Yallop, C., Fletcher, J. (2007). An Introduction to Phonetics and Phonology. Oxford: Blackwell.