How big is a heart? Two-fifths of a land rod of course, according to Aztec surveyors, who measured land to figure tax and tribute.

The Aztecs had more numbers than we do, or at least symbols denoting numerical concepts. When it came to measuring land-critical for levying the proper tax or tribute-these medieval Mesoamericans used arrows, hearts, hands and other units representing fractions, according to a new study in Science.
To figure this out, mathematician Maria del Carmen Jorge y Jorge of the National Autonomous University of Mexico (U.N.A.M) channeled the mind of an Aztec land surveyor. That meant retraining herself to use a different numerical system and combing through the Codex Vergara, one of two remaining books that record Aztec land surveying.