@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},
     pages = {343--362},
     publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
     volume = {17},
     number = {4},
     year = {1976},
     mrnumber = {440553},
     zbl = {0344.18004},
     language = {en},
     url = {http://www.numdam.org/item/CTGDC_1976__17_4_343_0/}
}