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Arduino compatible boards are at minimum, hardware-compatible with certain Arduino shields based upon the header, or pin-out connector. Most newer “Arduino compatible” boards claim to be compatible with the same shields that the Arduino UNO Rev 3 board is compatible with. The UNO Rev 3 has more pins than earlier Arduino boards, including an IO REF pin that is used to determine at what supply voltage (3.3 or 5V) a shield requires. Shields are boards that can be plugged on top of the Arduino PCB to extend its capabilities. The different shields follow the same philosophy as the original toolkit: they are easy to mount, and cheap to produce.