(A): Ancestral acceptor stems of tRNAs that have complementary second base pairs (shown in red), in accordance with dual complementarity (Table 1). The putative r-aaRSs are shown as wavy arrows. "S" stands for either G or C. These two acceptor stems could have existed during the earliest stages of code shaping, even predating translation (i.e. before the pairs of single-stranded anticodons at the bottom of Figure 3 came into existence). (B): The same acceptor stems as in (A), but with the mutually complementary GGC/GCC and GCC/GGC pairs in the center. "Prohibited" denotes more confusion-prone recognition of acceptors by r-aaRSs.