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On May 9, 2011, the first physical incarnation of bitcoin, known as Bitbills, were introduced. Bitbills were plastic cards that held the cryptographic key that would allow the face value amount of the card to be spent.
The key was on a QR code embedded within the card such that it could not be read without the card showing evidence of tampering.
Additionally, the card had a hologram affixed to ensure that the card was not a counterfeit.