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Adaptive immunity depends on diverse T cell receptor repertoires generated by variable, diversity, and joining (V[D]J) recombination. Here, we define the principles by which combinatorial diversity is generated in the murine Tcrarepertoire. Tcraand Tcrdgene segments share the Tcra-Tcrdlocus, with interspersed Vαand Vδsegments undergoing Vδ-Dδ-Jδrearrangement in CD4−CD8−thymocytes and then multiple rounds of Vα-Jαrearrangement in CD4+CD8+thymocytes. We document stepwise, highly coordinated proximal-to-distal progressions of Vαand Jαuse on individual Tcraalleles, limiting combinatorial diversity. This behavior is supported by an extended chromatin conformation in CD4+CD8+thymocytes, with only nearby Vαand Jαsegments contacting each other. Tcrdrearrangements can use distal Vδsegments due to a contracted Tcra-Tcrdconformation in CD4−CD8−thymocytes. These rearrangements expand the Tcrarepertoire by truncating the Vαarray to permit otherwise disfavored Vα-Jαcombinations. Therefore, recombination events at two developmental stages with distinct chromatin conformations synergize to promote Tcrarepertoire diversity.