The third complementarity-determining region (CDR) of the TCR α and β chains forms loops that engage amino acid residues of peptides complexed with MHC. This interaction is central to the specific discrimination of antigenic-peptide-MHC complexes by the TCR. The TCRβ chain CDR3 loop is encoded by the Dβ gene segment and flanking portions of the Vβ and Jβ gene segments. The joining of these gene segments is imprecise, leading to significant variability in the TCRβ chain CDR3 loop length and amino acid composition. In marked contrast to other pairing antigen-receptor chains, the TCRβ and α chain CDR3 loop size distributions are relatively narrow and closely matched. Thus, pairing of TCR α and β chains with relatively similar CDR3 loop sizes may be important for generating a functional repertoire of αβ TCR. Here we show that the TCRβ chain CDR3 loop size distribution is minimally impacted by TCRβ chain or αβ TCR selection during thymocyte development. Rather, this distribution is determined primarily at the level of variable-region gene assembly, and is critically dependent on unique features of the V(D)J recombination reaction that ensure Dβ gene segment utilization.