The language system comprises sets of units of different types, e.g. phonemes, morphemes, words etc. When such a unit U occurs in a text, it occupies a certain position in it and eo ipso stands in the intersection of two relational axes:

• U bears paradigmatic relations to other units (of the same type) which could occupy U's position in the text.
• U bears syntagmatic relations to other units (of the same type) neighboring on U in the same text.

Thus, a paradigmatic relation is a relation in absentia, while a syntagmatic relation is one in praesentia.

To each of the two axes, a fundamental operation of linguistic activity corresponds. A unit is selected by its paradigmatic relations and combined with other units by its syntagmatic relations. Thus, paradigmatic relations produce classes, syntagmatic relations produce constructions.