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, 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. The basic premise of internal reconstruction is that a meaning-bearing element that alternates between two or more similar forms in different environments was probably a single form in the past, into which alternation was introduced by the usual mechanisms of sound change and analogy.) Language forms reconstructed by means of internal reconstruction are denoted with the pre- prefix, similar to the use of proto- to indicate a language reconstructed by means of the comparative method; for example, proto-Indo-European.