A Characterization of Modularity for Congruence Lattices of Algebras*
Alan Day · Canadian Mathematical Bulletin · 1969
Let us call an equational class (variety) K of algebras permutable if and only if every pair of congruences on each K-algebra is permutable. Similarly, we will call K modular (distributive) if the congruence lattice of each K-algebra is modular (distributive).