A related, but weaker notion than the existence of normal forms is that of two objects being joinable: "x" and "y" are said to be joinable if there exists some "z" with the property that formula_43. From this definition, it is apparent that one may define the joinability relation as formula_44, where formula_45 is the composition of relations. Joinability is usually denoted, somewhat confusingly, also with formula_46, but in this notation the down arrow is a binary relation, i.e. we write formula_47 if "x" and "y" are joinable.