A gas turbine engine includes first and second arc segments that each have leading and trailing sides and first and second circumferential sides. The first circumferential side includes a first slot portion and the second circumferential side includes a second slot portion. The first and second slot portions are in registration and together define a seal slot. At least the first slot portion has a first concave arced back wall. A feather seal is entrapped in the seal slot and includes first and second feather seal circumferential sides. At least the first feather seal circumferential side has a first convex arced edge that is adjacent the first concave arced back wall of the first slot portion.