A blood clot is a meshwork of fibrin fibers built up by the systematic assembly of fibrinogen molecules proteolyzed by thrombin. Here, we describe a model of how the assembly process occurs. Five kinds of interaction are explicitly defined, including two different knob-hole interactions, an end-to-end association between gamma-chains, a lateral association between gamma-chains, and a hypothetical lateral interaction between beta-chains. The last two of these interactions are responsible for protofibril association and are predicated on intermolecular packing arrangements observed in crystal structures of fibrin double-D fragments cocrystallized with synthetic peptides corresponding to the knobs exposed by the release of the fibrinopeptides A and B.