Medieval pilgrimage produced souvenirs, and the souvenirs were the proof. A pilgrim who reached Rome came back with a keyed badge, one who reached Canterbury with an ampulla, and one who reached Compostela with a scallop shell. The Pilgrim's Guide, the fifth book of the Codex Calixtinus written in the twelfth century, describes the shells on sale in front of the cathedral, and the cathedral treated the right to sell them as a source of income worth defending. Shells were pierced and sewn to a hat or a bag, or cast in lead and pewter as cheaper copies, and the finished pilgrim was recognisable at a distance for the whole of the journey home.
The badge had legal weight as well as devotional value. Pilgrimage was undertaken as penance, sometimes imposed by a court or a confessor, and evidence of completion mattered. It also conferred protections: a recognisable pilgrim could claim hospitality and, in principle, safe passage. That is why the shells appear in graves. Excavations of medieval burials across Britain, the Low Countries, Germany and Scandinavia have recovered scallop shells laid with the dead, hundreds of miles from any water where the species lives, buried with people who wanted the fact of the journey to go into the ground with them.