QR code
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A QR code (abbreviated from Quick Response Code) is a two-dimensional matrix barcode.
Duel Terminal cards featured a QR code printed in the center of the card backing. These are not visible to the naked eye, and were used by Duel Terminal machines to reliably identify inserted cards.
QR codes were publicly displayed in the Tokyo Dome during The Bond of Duelists event in Yu-Gi-Oh! Duel Links and Yu-Gi-Oh! Master Duel; these codes provided Friendship Chains which in turn gave rewards for each Friendship Chain the player received and passed on, with an additional reward for receiving and passing on all four Friendship Chains.
