Self-discrimination, a critical but ill-defined molecular process programmed during thymocyte development, requires myriad pre–T cell receptors (preTCRs) and abTCRs. Using x-ray crystallography, we show how a preTCR applies the concave b-sheet surface of its single variable domain (Vb) to “horizontally” grab the protruding MHC a2-helix. By contrast, abTCRs purpose all six complementarity-determining region (CDR) loops of their paired VaVb module to recognize peptides bound to major histocompatibility complex molecules (pMHCs) in “vertical” head-to-head binding. The preTCR topological fit ensures that CDR3b reaches the peptide’s featured C-terminal segment for pMHC sampling, establishing the subsequent abTCR canonical docking mode. “Horizontal” docking precludes germline CDR1b- and CDR2b-MHC binding to broaden b-chain repertoire diversification before abTCR-mediated selection refinement. Thus, one subunit successively attunes the recognition logic of related multicomponent receptors. [ABSTRACT FROM AUTHOR]