Pterocarpans are compounds derived from (6a$R$,11a$R$)-6a,11a-dihydro-6$H$-[1]benzofuro[3,2-$c$][1]benzopyran (15), and their 6a,11a-didehydro derivatives 16 are named coumestans. They are numbered systematically following the numbering rules for fused ring systems.