Lexicon is usually a list of words together with additional word-specific information, i.e., a dictionary. Lexicon is a word of Greek origin (λεξικόν) meaning vocabulary. When linguists study the lexicon, they study such things as what words are, how the vocabulary in a language is structured, how people use and store words, how they learn words, the history and evolution of words, types of relationships between words as well as how words were created. The term is also sometimes used in the title of an encyclopaedic dictionary or an encyclopedia, especially for 19th century works and those written in German (lexikon). In linguistics, lexicon has a slightly more specialised definition, as it includes the lexemes used to actualize words. Lexemes are formed according to morpho-syntatcic rules and express sememes. In this sense, a lexicon organises the mental vocabulary in a speaker's mind: First, it organises the vocabulary of a language according to certain principles (for instance, all verbs of motion may be linked in a lexical network) and second, it contains a generative device producing (new) simple and complex words according to certain lexical rules. For example, the suffix '-able' can be added to transitive verb only such that we get 'read-able' but not '*cry-able'. Furthermore an individual's lexical knowledge (or lexical concept) is that person's knowledge of vocabulary.