But each word will also have relationships with other words in the language that do not occur at this point in time, but are capable of doing so. The word, that is, has “formulaic” associations with those other words from among which it has, so to speak, been chosen. And these other words, “part of the inner storehouse that makes up the language of each speaker” (Saussure, p. 123)—they might be synonyms, antonyms, words of similar sounds or of the same grammatical function—help, by not being chosen, to define the meaning of the word which has. It obviously follows from our notion of language as a self-contained structure that the absence of certain word partly creates and certainly winnows and refines the meanings of those that are present, and in the sentence “the boy kicked the girl”, part of the meaning of “kicked” derives from the fact that it turns out not to be “kissed” or “killed” as the full relationships of the words in the sentence are unrolled. These kinds of relationships can be thought of as on a “vertical” plane to distinguish them from the simultaneously operating yet quite distinct relationships of the horizontal, syntagmatic plane [i.e., a word’s relationships with the other words that precede and follow it in a sentence]. They constitute the word’s associative aspect, and obviously form parts of its “synchronic” relationship with the whole language structure.

adapted from Terence Hawkes, Structuralism and Semiotics (University of California Press, 1977), p. 27