Interconnecting modular connectors or headers have flexible tongue and groove structures for forming a variety of header assembly configurations. The interconnecting modular headers can have locking structures in addition to the tongue and groove mating structures for permanently interlocking the modular headers together. The sidewalls of each modular connector can be parallel to each other and disposed at supplementary angles with respect to the rear wall of the housing. The top wall of each modular connector can have a portion wall extending inwardly from each sidewall at an angle of greater than ninety degrees and each portion wall can have an upwardly extending wall that joins the portion walls to a ceiling. The portion walls can join with the upwardly extending walls to form a T-shape corner. The upwardly extending walls can join with the ceiling equally at an angle of greater than ninety degrees.