“[…] for most of the Middle Ages the ♥ as a shape held no specific link with the organ of the heart. ♥s are certainly found on all sorts of medieval objects, but simply as one decorative motif among many alongside vine scrolls, hatched lines, chequerboard patterns or circular spirals. It is still unclear precisely how this formalised yet abstract shape actually came to stand for the organ and emotions of the heart as it does now. Perhaps it was an association with the leaves of creeping ivy or other plants thought at the time to be aphrodisiacs. Or perhaps the pre-existing ornamental ♥ shape simply grew quite naturally to fit descriptions of a bicameral organ, more pointed at one end than the other. Either way, the use of this symbol seems to have concretised only towards the very end of the medieval period, when it featured in several early European printed images. The new technology of print meant that from the 1450s onwards images could be created far more easily and spread far more quickly across different regions and audiences. This broad circulation seems to have been the thing to seal common agreement that the ♥ symbol represented the heart, especially its amorous properties. From these novel and newly abundant images, especially printed in cards, the sign began quickly appearing on all manner of late medieval and early renaissance objects.”
— Jack Hartnell, Medieval Bodies: Life, Death and Art in the Middle Ages