We have spent some time in this chapter contrasting the dictionary with the mental lexicon in order to understand the question “Is xyz really a word?” We are now in a position to understand this question better, at least from the point of view of morphologists. Most morphologists would say that xyz is a word if it can be formed by the rules of word formation in a particular language. So words like wordhood or re-reprise that you might never have seen before you read this chapter really are words, even though you won’t find them in any dictionary. They are words because they follow the rules of English word formation. It is the rules of word formation that we know that most distinguish our mental lexicon from the dictionary. The dictionary does not need to list all the words that we know or that we could create, because once we know word formation rules we can produce and understand potentially infinite numbers of new words from the morphemes available to us. The remainder of this book will be an attempt to work out in some detail what those rules are.