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We give two related universal properties of the span construction. The first involves sinister morphisms out of the base category and sinister transformations. The second involves oplax morphisms out of the bicategory of spans having an extra property; we call these `jointed' oplax morphisms.
@article{TAC_2004_13_a3, author = {R.J.MacG. Dawson and R. Pare and D.A. Pronk}, title = {Universal properties of {Span}}, journal = {Theory and applications of categories}, pages = {61--85}, publisher = {mathdoc}, volume = {13}, year = {2004}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2004_13_a3/} }
R.J.MacG. Dawson; R. Pare; D.A. Pronk. Universal properties of Span. Theory and applications of categories, The Carboni Festschrift, Tome 13 (2004), pp. 61-85. http://geodesic.mathdoc.fr/item/TAC_2004_13_a3/