Hilton, P. (1966). Correspondences and Exact Squares. In S. Eilenberg, D. K. Harrison, S. MacLane, & H. Röhrl (Eds.), Proceedings of the Conference on Categorical Algebra (pp. 254–271). Springer Berlin Heidelberg. 
@inproceedings{hilton-exacts,
  title = {Correspondences and Exact Squares},
  author = {Hilton, Peter},
  year = {1966},
  booktitle = {Proceedings of the Conference on Categorical Algebra},
  publisher = {Springer Berlin Heidelberg},
  address = {Berlin, Heidelberg},
  pages = {254--271},
  isbn = {978-3-642-99902-4},
  editor = {Eilenberg, S. and Harrison, D. K. and MacLane, S. and R{\"o}hrl, H.}
}