The rules of a language, also called grammar, are learned as one acquires a language. These rules include phonology, the sound system, morphology, the structure of words, syntax, the combination of words into sentences, semantics, the ways in which sounds and meanings are related, and the lexicon, or mental dictionary of words. When you know a language, you know words in that language, i.e. sound units that are related to specific meanings. However, the sounds and meanings of words are arbitrary. For the most part, there is no relationship between the way a word is pronounced (or signed) and its meaning.