This document provides an introduction to lexico-grammar, which examines the interdependency of vocabulary and grammatical structures. It describes how language meets the demand of having infinite meanings from finite expression units through an intermediate level of lexico-grammar. Lexico-grammar provides means to arrange grammatical structures to make different meanings, such as extending language through coining new words or using structures atypically. Constituents in a clause take on multifunctional roles to simultaneously realize ideational, interpersonal and textual meanings.