Interlanguage, the learner's developing grammar of the second language, is highly systematic. Learners from different language backgrounds follow similar developmental sequences.
However, interlanguages are also much more unstable than L1 grammars, as Romaine's definition implies:
The variable language systems of increasing complexity that develop during the process of acquiring a second language.
The study of interlanguage variation can be dated from Tarone (1983). Research shows that interlanguages vary across and within learners, and at a given point in time as well as over longer periods.
Variation across learners
Inter-learner variation is apparent when we compare rates of acquisition and ultimate attainment in groups of learners. Even with comparable levels of exposure to the target language, some learners will progress rapidly, others more slowly. Factors which play a role in interlanguage development include age, motivation, and aptitude.
Variation over time
A given learner will create a series of overlapping grammars of the target language as he or she progresses closer to a native variety. In early stages, the interlanguage may resemble the L1 grammar, but later move closer to the L2 grammar of the native speaker. These changes constitute diachronic variation.
However, at a given moment, interlanguage production may show also synchronic variability. Learners' performances vary depending on contextual variables such as mode (spoken or written), topic, or interlocutor. Learners may make more errors in stressful exam contexts, for example, or speak more fluently in the company of their peers.