An old book written by hand, with pages you turn, not a long scroll you unroll. Before printing, people copied laws, holy texts, and science this way, often onto animal skin. Scholars still say “codex” for a famous old copy — a named manuscript sitting in a library — not for the paperback in your bag. It is a museum and university word. Latin cōdex first meant a tree trunk, then a wooden writing tablet, then a book; English “code” is a doublet of that same Latin word, which is why the two look related, though actually one stayed with books and the other drifted toward rules and signals. In plant science you may still meet caudex for a thick stem, the other Latin shape of the same word.