Abstract: A connector for attaching a beam of a modular structure to either a post or another beam whereby the number of different types of parts in inventory may be reduced. Tabs on an edge of the connecter engage longitudinal grooves formed in a beam surface and are laterally located. The tabs alternatively engage slots sized to a tab width in a wall of a post for precise attachment thereto. A pin is spring biased to protrude from the edge outwardly to engage an aperture in the beam or post to prevent upward movement of and thereby lock the connector thereto whereby the connector may be attached quickly and easily and without inadvertently causing already attached connectors to be disengaged from other walls of a post.