By the mental lexicon we mean the sum total of everything an individual speaker knows about the words of language. This knowledge includes information about pronunciation, category (part of speech), and meaning, of course, but also information about syntactic properties (for example, whether a verb is transitive or intransitive), level of formality, and what lexicographers call ‘range of application’, that is, the specific conditions under which we might use the word. For example, I know that the word verandah is a noun, pronounced (in my American English) [vəɹændə],1 that it refers to a type of porch, and that I’d only use it in reference to the sort of porch one finds in the southern part of the US or perhaps in some exotic tropical country. Unless I was being ironic, I probably would not call my own back porch ‘the verandah’. I also know that barf is a verb that’s pronounced [ba r f], that it means ‘vomit’, that it is intransitive (unless used with a particle like up) and that it is used only colloquially (I wouldn’t use it if I were describing the symptoms of a stomach flu to the doctor).

It is quite likely that in our mental lexicons we have entries that are only partial. We may know the pronunciation of a word, but not its meaning (e.g., I know how to pronounce amortize, but I’m not sure what it means). Or the opposite: for example, I know what the word hegemony means, but I don’t know if it’s pronounced with the stress on the first or second syllable. We may also have only partial knowledge of the meaning of a word. I know, for example, that a distributor is part of a car and that if you have to replace it, it’s a relatively expensive job, but I don’t know what a distributor looks like or what it does.

Each person’s mental lexicon is sure to contain things that are different from other people’s mental lexicons. One person may know lots of words for types of birds or flowers, another might know all the specialized vocabulary of sailing, and so on. Auto mechanics surely know more details of the meaning of the word distributor than we do. But our individual mental lexicons overlap enough that we speak the same language. In this section we will look in more detail at the contents of our mental lexicons, both what is stored and what is created by rules of word formation, and how our mental lexicons are organized.