The document defines inter-language as the language system produced by second and foreign language learners who are in the process of learning a new language. Inter-language develops based on rules from the learner's first language and the target language, and may not reflect features of either. The inter-language system changes over time as rules are altered, deleted, or added. Learners progress through stages from early approximations of the target language to later intermediate and final stages. Fossilization occurs when errors become impossible to correct despite ability and motivation.