“…Analogous results have been obtained for many other important families of semigroups, such as full linear monoids [26,29,38], endomorphism monoids of various algebraic structures [15,18,30,31,37], sandwich semigroups [13,14] and, more recently, certain families of diagram monoids [22,24,25,58].The current article mainly concerns the partial Brauer monoid PB n and the Motzkin monoid M n (see Section 2 for definitions). There is a growing body of literature on partial Brauer monoids and algebras; 1 see for example [7,16,43,46,55,60,[64][65][66]. Motzkin monoids and algebras are a more recent phenomenon [7,9,17,46,66,70], and are planar versions of the partial Brauer monoids and algebras (see Section 2); the relationship between PB n and M n is therefore analogous to that between the Brauer monoid B n and the Jones monoid J n .…”