A ternary algebra M whose ternary operation (abc) satisfies the identities and for all is called a median algebra. Several papers have studied tolerances, reflexive and symmetric compatible relations on algebras. These relations are well understood for distributive lattices, median algebras, and tree algebras (median algebras in which any (abd), (acd), (bcd) are not distinct).
This paper gives the main facts, provides simple proofs, and extends some previous results concerning tolerances. The following are typical results. If is a reflexive, symmetric relation on a tree algebra, then is a tolerance if and only if all blocks of are convex. A median algebra has the tolerance extension property if and only if it is a tree algebra. The lattice of all tolerances on a median algebra is distributive.