1. M. Artin, A. Grothendieck, J.-L. Verdier, Théorie des Topos, SGA 4, Second Edition, Springer Lecture Notes in Math. 269 and 270, 1972
2. M. Barr, Non-abelian full embedding: outline. Actes, Congrès International, 1 (1970) 309–312.
3. M. Barr, Toposes without points, J. Pure Appi. Algebra 5 (1974) 265–280.
4. J. Bénabou, On a formal language for topos theory, unpublished lecture, University of Montreal, April 1973.
5. G. Birkhoff, Subdirect Unions in Universal Algebra, Bull. Amer. Math. Soc. 50 (1944) 764–768.