Biochemistry

The twelfth entry is le repliement des protéines, and the noun in the middle of it is an old acquaintance of English wearing a lab coat. Plier is to fold, as one folds a map; replier is to fold it again or fold it in, and it is also what an army does when it pulls back — replier ses troupes, un repli stratégique. Behind both stands Latin replicare, to fold again. Now the surprise. « To reply » is that same verb, taken into English from Old French replier in the fourteenth century, when folding a letter back and turning an argument back on its author were one gesture. « A replica » arrived later by way of Italian, and « to replicate » straight from the Latin. So a reader who answers a letter, copies an experiment and watches a chain of amino acids collapse into its shape is using one verb three times over. Modern French keeps replier for the folding alone: for answering it says répondre, and répliquer is to retort. Read le repliement as a reply and the protein starts answering back.