The Nicene Creed as the councils issued it says that the Holy Spirit proceeds from the Father. Spanish and then Frankish churches had come to recite it with an added word, Filioque, and from the Son, and by the end of the eighth century the addition was normal in Charlemagne's chapel at Aachen. In 809 Frankish monks in Jerusalem were accused by Greek monks of heresy for singing the creed that way, and the dispute came to Rome. A synod at Aachen approved the addition and sent envoys to ask the pope to authorize it for the whole church.
Leo III gave an answer of unusual precision. He told the Frankish envoys that the doctrine the word expressed was sound and could be taught, and that the text of the creed itself must not be altered, because a council had settled its wording and no local church had authority to add to it. To make the ruling permanent he had the creed engraved in Greek and Latin, without the Filioque, on two silver shields and set up at the confession of Saint Peter, where anyone could read what Rome actually recited.
The distinction he drew, between a true teaching and an authorized text, held in Rome for about two centuries. The Filioque entered the creed at Rome only in the eleventh century, and by then it had become one of the standing grievances of the Eastern churches against the Western, cited in the mutual excommunications of 1054 and in every reunion negotiation since. Leo's shields are long gone, but his ruling is still quoted in ecumenical discussion as evidence that the Roman see once distinguished the two questions.