of language, similar to speech by children or in pidgins. This may be related to linguistic universals. The concept of interlanguage has become very widespread in SLA research and is often a basic assumption made by researchers. Sequences in the acquisition of English inflectional morphology In the 1970s, several studies investigated the order in which learners acquired different grammatical structures. These studies showed that there was little change in this order among learners with different first languages. Furthermore, it showed that the order was the same for adults and children and that it did not even change if the learner had language lessons. This supported the idea that there were factors other than language transfer involved in learning second languages and was a strong confirmation of the concept of interlanguage. However, the studies did not find that the orders were the same. Although there were remarkable similarities in the order in which all learners learned second-language grammar, there were still some differences between individuals and learners with different first languages. It is also difficult to tell when exactly a grammatical structure has been learned, as learners may use structures correctly in some situations but not in others. Thus it is more accurate to speak of sequences of acquisition, in which specific grammatical features in a language are acquired before or after certain others but the overall order of acquisition is less rigid. For example, if neither feature B nor feature D can be acquired until feature A has been acquired (feature B and D depend on A) and feature C depends on B, but D does not depend on B (or, therefore, on C), then acquisition orders (A, B, C, D) and (A, D, B, C) are possible, as they are both valid topological orderings. Learnability and teachability Learnability has emerged as a theory explaining developmental sequences that crucially depend on learning principles, which are viewed as fundamental mechanisms of language acquisition within learnability theory. Some examples of learning principles include the uniqueness principle and the subset principle. The uniqueness principle refers to learners' preference for a one-to-one mapping between form and meaning, while the subset principle posits that learners are conservative in that they begin with the narrowest hypothesis space that is compatible with available data. Both of these principles have been used to explain children's ability to evaluate grammaticality despite the lack of explicit negative evidence. They have