Transfer RNA's are probably very strongly selected for translational efficiency. In this article, the argument is presented that the coding performance of the triplet anticodon is enhanced by selection of a matching anticodon loop and stem sequence. the anticodon plus these nearby sequence features (the extended anticodon) therefore contains more coding information than the anticodon alone and can perform more efficiently and accurately at the ribosome. This idea successfully accounts for the relative efficiencies of many transfer RNA's.