The third complementarity-determining region (CDR) of the TCR alpha and beta 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 TCRbeta chain CDR3 loop is encoded by the Dbeta gene segment and flanking portions of the Vbeta and Jbeta gene segments. The joining of these gene segments is imprecise, leading to significant variability in the TCRbeta chain CDR3 loop length and amino acid composition. In marked contrast to other pairing antigen-receptor chains, the TCR beta and alpha chain CDR3 loop size distributions are relatively narrow and closely matched. Thus, pairing of TCR alpha and beta chains with relatively similar CDR3 loop sizes may be important for generating a functional repertoire of alpha beta TCR. Here we show that the TCRbeta chain CDR3 loop size distribution is minimally impacted by TCRbeta chain or alpha beta 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 Dbeta gene segment utilization.