Double groupoids and crossed modules
Cahiers de Topologie et Géométrie Différentielle Catégoriques, Volume 17 (1976) no. 4, p. 343-362 
@article{CTGDC_1976__17_4_343_0,
     author = {Brown, Ronald and Spencer, Christopher B.},
     title = {Double groupoids and crossed modules},
     journal = {Cahiers de Topologie et G\'eom\'etrie Diff\'erentielle Cat\'egoriques},
     publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
     volume = {17},
     number = {4},
     year = {1976},
     pages = {343-362},
     zbl = {0344.18004},
     mrnumber = {440553},
     language = {en},
     url = {http://www.numdam.org/item/CTGDC_1976__17_4_343_0}
}

Brown, Ronald; Spencer, Christopher B. Double groupoids and crossed modules. Cahiers de Topologie et Géométrie Différentielle Catégoriques, Volume 17 (1976) no. 4, pp. 343-362. http://www.numdam.org/item/CTGDC_1976__17_4_343_0/

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