What is “a seal” on a medieval document used for?

The story behind the answer

Medieval charters and letters carried wax impressions made with a ruler’s seal matrix or signet ring to show the document truly came from that person or office. The distinctive design—often heraldic—served like a unique signature, and the wax also made tampering visible; a broken or mismatched impression warned readers not to trust the text.

Seals came in colored waxes (red, green, natural) and different materials: nobles used wax, while popes issued lead seals called bullae. Because many people were illiterate and written signatures weren’t standardized, the image on a pendant or applied seal conveyed identity and authority at a glance. Dates and taxes were recorded elsewhere; the seal’s job was to guarantee origin and integrity.