A ceramic matrix composite (CMC) airfoil assembled from a pressure side wall (42) and a suction side wall (52) joined by interlocking joints (18, 19) at the leading and trailing edges (22, 24) of the airfoil to produce a tapered thin trailing edge. The trailing edge (24) is thinner than a combined thicknesses of the airfoil walls (42, 52). One or both of the interlocking joints (18, 19) may be formed to allow only a single direction of assembly, as exemplified by a dovetail joint. Each joint (18, 19) includes keys (44F, 54F, 56F, 46F) on one side and respective keyways (44K, 54K, 56K, 46K) on the other side. Each keyway may have a ramp (45) that eliminates indents in the airfoil outer surface that would otherwise result from the joint.