Q-systems are a method of directed graph transformations according to given grammar rules, developed at the Université de Montréal by Alain Colmerauer in 1967–70 for use in natural language processing.
The Université de Montréal's machine translation system, TAUM-73, used the Q-Systems as its language formalism.
An extension of the basic idea of the Q-Systems, namely to replace instantiation by unification (to put it simply, allow "new" variables in the right hand side part of a rule, and replace parametrized labelled trees by logical terms) led to Prolog, designed by Alain Colmerauer and Philippe Roussel in 1972.
Refinements in the other direction (reducing non-determinism and introducing typed labels) by John Chandioux led to GramR, used for programming METEO from 1985 onward.
In 2009, Hong Thai Nguyen of GETALP,[1] Laboratoire d'Informatique de Grenoble[2] reimplemented the Q-language in C, using ANTLR to compile the Q-systems and the Q-graphs, and an algorithm proposed by Christian Boitet (as none had been published and sources of the previous Fortran implementation had been lost).