On close inspection, the language produced by L2 learners contains a large number of “errors” that seem to have no connection to the forms of either the L1 or L2. For example, the Spanish L1 speaker who says in English She name is Maria is producing a form that is not used by adult speakers of English, does not occur in English L1 acquisition by children, and is not based on a structure in Spanish. Evidence of this sort suggests that there is some in-between system used in the L2 acquisition process that certainly contains aspects of the L1 and L2, but which is an inherently variable system with rules of its own. This system is called an interlanguage and it is now considered to be the basis of all L2 production.

If some learners develop a fairly fixed repertoire of L2 expressions, containing many forms that do not match the target language, and seem not to be progressing any further, their interlanguage is said to have “fossilized.” The process of fossilization in L2 pronunciation seems to be the most likely basis of what is perceived as a foreign accent. However, an interlanguage is not designed to fossilize. It will naturally develop and become a more effective means of L2 communication given appropriate conditions. Discovering just what count as the appropriate conditions for successful L2 learning is an ongoing area of investigation.