When we analyze sentences by looking into their constituents and generalizing the pattern, we start from the perspective of form. We actually look at the chain relations (also called syntagmatic relations) of the constituents, overlooking another type of relations which are termed choice relations (also termed paradigmatic relations). If a linguist is interested in language in use rather than language as an abstract system, he/she will explore choice relations and endeavors to find what are the options and what factors affect the speakers' choices. Consider (51):

　 (51) Have you seen a cat?

Each constituent, which occupies a slot in the sentence, is chosen from among all the members of a syntactic category. The article a, for example, is selected from a class of words, including, the, my, your, our, that, this, etc. All these can occupy the slot without harming the grammaticality of the sentence. However, when we ask this question in communication, only one of them is appropriate. The choice depends on the context, especially on who you are talking to. If you ask a stranger, (51) is the right sentence. If you ask your family member, you will say, “Have you seen the cat?” If you ask your neighbor, you will say “Have you seen my/our cat?”

　 Systemic-Functional Grammar starts from the perspective of function and meaning in analyzing sentences. It views language as a system of meaning potentials and sentence structure as the result of a process of selection. The grammatical system of any language is a system of options. The form of language is the realization of function. The task of functional grammar is to account for the intricate relation of function to form.