A central role of the lexicon is the documenting of established lexical norms and conventions. Lexicalization is the process in which new words, having gained widespread usage, enter the lexicon. Since lexicaliztation may modify lexemes phonologically and morphologically, it is possible that a single etymological source may be inserted into a single lexicon in two or more forms. These pairs, called a doublet, are often close semantically. Two examples are aptitude versus attitude and employ versus imply.