Synchronic and diachronic approaches can come - equally correctly - to quite different conclusions. For example, a Germanic strong verb like English sing - sang - sung is irregular when viewed synchronically: the native speaker's brain processes these as learned forms, whereas the derived forms of regular verbs are processed quite differently, by the application of productive rules. This is an insight of psycholinguistics, relevant also for language didactics, both of which are synchronic disciplines. However a diachronic analysis will show that the strong verb is the remnant of a fully regular system of internal vowel changes; historical linguistics seldom uses the category "irregular verb".