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Onion rings is the name of an agar of density ½ whose unit cell is composed of nested squares, which are fancied as resembling the rings of an onion.
A family of agars
Onion rings can be thought of as one specific member of a family of agars whose unit cells are 4n × 4n for a given n>1, further subdivided into quadrants; for the canonical onion rings agar, n is equal to 3.
In each agar, the NE and SW quadrants contain onions with a block core, while the NW and SE quadrants contain complementary onions. This arrangement inhibits all straight edges from sprouting live cells; each member of the family is a still life covering the entire infinite plane.
A few different members of the extended onion rings family of agars are shown below: