Scrubber is a period-2 oscillator that was found in 1971. It is a billiard table configuration with a 4-cell on-off rotor. If its nucleus, a ring of four triminoes, is not constrained it will expand after two generations into a traffic light. Two covers are the smallest option for induction coils.
Placing two nuclei side by side is a form of constraint. Separated by the width of a single cell they can be packed rather arbitrarily into a square lattice, for which an overall constraint is still necessary. (Such an arrangement is shown at Agar#Tile_agars.) The interiors of nuclei separated by the width of two cells are still protected, but the nuclei act like spark coils. Glasses constitutes a binary instance of the arrangement, which can be elaborated as much as desired within a square lattice. The overall figure has period 2, but the relation between the interior and exterior phase must be respected. Boundary constraint is still required. Incommensurability of the widths would preclude mixing single and double spacing except along a linear chain which could even be bounded by spark coils.
In other rules
The scrubber can exhibit at least eight different evolution sequences across certain non-totalistic rules, allowing it to have any period from at least period 1 to period 8.
- 28P2.17076 at Heinrich Koenig's Game of Life Object Catalogs