Phistomefel Ring

The Phistomefel Ring (also referred to as Phistomefel's Ring and Phistomefel's Theorem in some cases) is a form of Set Equivalence Theory that states the 2x2 corner blocks of any Regular 9x9 Sudoku Grid contain exactly the same digits as the cells that border the edge of box 5 (including the corners). It was popularized by Phistomefel in April 2020 and has gained lots of attention in the world of sudoku. Many sudoku constructors have since used this property to create puzzles with interesting break-ins.