Words and lexemes, types and tokens

How many words occur in the following sentence?

My friend and I walk to class together, because our classes are in the same building and we dislike walking alone.

You might have thought of at least two ways of answering this question, and maybe more. On the one hand, you might have counted every item individually, in which case your answer would have been 21. On the other hand, you might have thought about whether you should count the two instances of and in the sentence as a single word and not as separate words. You might even have thought about whether to count walk and walking or class and classes as different words: after all, if you were not a native speaker of English and you needed to look up what they meant in the dictionary, you’d just find one entry for each pair of words. So when you count words, you may count them in a number of ways.

Again, it’s useful to have some special terms for how we count words. Let’s say that if we are counting every instance in which a word occurs in a sentence, regardless of whether that word has occurred before or not, we are counting word tokens. If we count word tokens in the sentence above, we count 21. If, however, we are counting a word once, no matter how many times it occurs in a sentence, we are counting word types.

Counting this way, we count 20 types in the sentence above: the two tokens of the word and count as one type. A still different way of counting words would be to count what are called lexemes. Lexemes can be thought What is morphology? 5 of as families of words that differ only in their grammatical endings or grammatical forms; singular and plural forms of a noun (class, classes), present, past, and participle forms of verbs (walk, walks, walked, walking), different forms of a pronoun (I, me, my, mine) each represent a single lexeme. One way of thinking about lexemes is that they are the basis of dictionary entries; dictionaries typically have a single entry for each lexeme. So if we are counting lexemes in the sentence above, we would count class and classes, walk and walking, I and my, and our and we as single lexemes; the sentence then has 16 lexemes.