Fluid construction grammar (FCG) is an open-source computational construction grammar formalism that allows computational linguists to formally write down the inventory of lexical and grammatical constructions as well as to do experiments in language learning and language evolution. FCG is an open instrument that can be used by construction grammarians who want to formulate their intuitions and data in a precise way and who want to test the implications of their grammar designs for language parsing, production and learning.

FCG integrates many notions from contemporary computational linguistics such as feature structure and unification-based language processing. Rules are considered bi-directional and hence usable both for parsing and production. Processing is flexible in the sense that it can even cope with partially ungrammatical or incomplete sentences. FCG is called 'fluid' because it acknowledges the premise that language users constantly change and update their grammars. 

FCG provides a framework for bi-directional parsing and production. But aside from reserving the 'top' unit name and a few feature names, FCG makes no commitments to any grammatical theory of word categories, and makes no commitment to any particular theory of grammatical constructions. Users of FCG make these choices by writing FCG grammar rules accordingly.

Fluid Construction Grammar can be downloaded as part of the Babel 2 software package. Babel 2 is a general cognitive framework that connects FCG with language technologies for conceptualization and interpretation. Download Babel 2.