21P2 is a period 2 oscillator.
21P2 is about the thirty-eighth most common naturally-occurring oscillator in Achim Flammenkamp's census, 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.