Definition of Internal reconstruction

Internal reconstruction is a method of recovering information about a language's past from the characteristics of the language at a later date. Whereas the comparative method compares variations between languages — such as in sets of cognates — under the assumption that they descend from a single proto-language form, internal reconstruction compares variant forms within a single language under the assumption that they descend from a single, regular form. For example, these could take the form of allomorphs of the same morpheme.