Until recently, in the study and description of vocabulary, the single word was widely considered to be the basic unit of meaning. There is no denying that single words form a substantial part of the vocabulary of English and that the word is perceived in language teaching as the basic unit to be acquired. Words, after all, carry important grammatical characteristics such as the ability to show number, person, tense, word class, etc.
Units consisting of more than one word, such as phrasal verbs, compounds and idioms, are often treated as items belonging to higher levels of proficiency. There are, of course, exceptions to this: everyday expressions (How are things?), common prepositional phrases (at the weekend), and high-frequency compounds (bus stop) are generally taught and picked up even at very elementary levels. The single word has served us well, and will continue to do so. But linguists have also, for a long time, sought out to understand how groupings of more than one word often have unitary meanings and specialised functions.
In other words, vocabulary knowledge involves not only knowing about an individual word; to a great extent, it involves knowing the words that, with very high frequency, tend to co-occur with it.
(O’KEEFFE, A., MCCARTHY, M. & CARTER, R. From corpus to classroom. Language Use and Language Teaching. Cambridge, 2007)
