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21P2 is a period 2 oscillator. It is about the 38th most common naturally-occurring oscillator, being less common than boat on spark coil and griddle and block, about as common as the fore and back, and more common than trans-block and long hook eating tub and trans-beacon down and long hook.
- Achim Flammenkamp (September 7, 2004). "Most seen natural occurring ash objects in Game of Life". Retrieved on January 15, 2009.