Two expressions or variants of a construction may be in free variation. This means that they are fully synonymous, there are no conditions in the language that determine their choice. The “language” here may be taken in a strict sense, limited to the language system (including its semantics), or in a broader sense, comprising style, pragmatics and extra-linguistic factors, too. With the broader sense, free variation is a rare phenomenon indeed. Normally there is some pragmatic, stylistic, sociolectal, dialectal or idiolectal factor which conditions a certain choice made by the speaker. Such conditions, however, are not generally an object for a descriptive grammar. Therefore the concept of free variation is generally restricted to freedom from conditions of the language system.

From a methodological point of view, a situation of free variation reflects a certain stage of an analysis, typically an incipient stage. If confronted with variants of a construction, an informant will often respond: they are the same; it makes no difference. It is then the linguist's task to find out about the contexts in which the variants are used. This search continues until the factors determining the use of the variants appear and the case thereby ceases to be one of free variation. In this methodological sense, a statement of free variation in a grammar amounts to the linguist throwing in the towel.