A connector assembly (10, 10A-G) comprising a first connector (20, 20A-G) and a second connector (50, 50A, 50E, 50G), wherein: the first connector (20, 20A-G) and the second connector (50, 50A, 50E, 50G) are mateable with each other along a mating direction; the first connector (20, 20A-G) comprises a first holding member (22, 22B, 22D), two or more first terminals (26, 26B, 26D) and a first shell (30, 30A-G); the first terminals (26, 26B, 26D) are held by the first holding member (22, 22B, 22D); each of the first terminals (26, 26B, 26D) has a first contact portion; the first shell (30, 30A-G) has a first mateable portion (32, 32A-G) and a first key (42, 42A); the first mateable portion (32, 32A-G) encloses, at least in part, the first contact portions in a perpendicular plane perpendicular to the mating direction; the first key (42, 42A) is a projection provided on the first mateable portion (32, 32A-G); the second connector (50, 50A, 50E, 50G) comprises a second holding member, two or more second terminals, a second shell and a housing (70, 70A, 70E, 70G); the second terminals are held by the second holding member; each of the second terminals has a second contact portion; the second shell has a second mateable portion (62, 62A); the second mateable portion (62, 62A) encloses all around the second contact portions in the perpendicular plane; the housing (70, 70A, 70E, 70G) has a projecting portion (74, 74A, 74E, 74G), a projecting end and a second key (744, 744A); the projecting portion (74, 74A, 74E, 74G) is apart from the second mateable portion (62, 62A) in the perpendicular plane and encloses, at least in part, the second mateable portion (62, 62A) in the perpendicular plane; the projecting end faces the first connector (20, 20A-G) in the mating direction when the first connector (20, 20A-G) and the second connector (50, 50A, 50E, 50G) are under a separated state where they are separated from each other in the mating direction; the second key (744, 744A) is a groove and extends along the mating direction; the first contact portions are brought into contact with the second contact portions, respectively, under a mated state where the first connector (20, 20A-G) and the second connector (50, 50A, 50E, 50G) are mated with each other; and under the mated state, the second mateable portion (62, 62A) is received in the first mateable portion (32, 32A-G), and the first key (42, 42A) is received in the second key (744, 744A), wherein the first key (42, 42A) is located outward of the first mateable portion (32, 32A-G) in the perpendicular plane; and the second key (744, 744A) is formed in the housing (70, 70A, 70E, 70G) and extends from the projecting end. the first connector (20, 20A-G) and the second connector (50, 50A, 50E, 50G) are mateable with each other along a mating direction; the first connector (20, 20A-G) comprises a first holding member (22, 22B, 22D), two or more first terminals (26, 26B, 26D) and a first shell (30, 30A-G); the first terminals (26, 26B, 26D) are held by the first holding member (22, 22B, 22D); each of the first terminals (26, 26B, 26D) has a first contact portion; the first shell (30, 30A-G) has a first mateable portion (32, 32A-G) and a first key (42, 42A); the first mateable portion (32, 32A-G) encloses, at least in part, the first contact portions in a perpendicular plane perpendicular to the mating direction; the first key (42, 42A) is a projection provided on the first mateable portion (32, 32A-G); the second connector (50, 50A, 50E, 50G) comprises a second holding member, two or more second terminals, a second shell and a housing (70, 70A, 70E, 70G); the second terminals are held by the second holding member; each of the second terminals has a second contact portion; the second shell has a second mateable portion (62, 62A); the second mateable portion (62, 62A) encloses all around the second contact portions in the perpendicular plane; the housing (70, 70A, 70E, 70G) has a projecting portion (74, 74A, 74E, 74G), a projecting end and a second key (744, 744A); the projecting portion (74, 74A, 74E, 74G) is apart from the second mateable portion (62, 62A) in the perpendicular plane and encloses, at least in part, the second mateable portion (62, 62A) in the perpendicular plane; the projecting end faces the first connector (20, 20A-G) in the mating direction when the first connector (20, 20A-G) and the second connector (50, 50A, 50E, 50G) are under a separated state where they are separated from each other in the mating direction; the second key (744, 744A) is a groove and extends along the mating direction; the first contact portions are brought into contact with the second contact portions, respectively, under a mated state where the first connector (20, 20A-G) and the second connector (50, 50A, 50E, 50G) are mated with each other; and under the mated state, the second mateable portion (62, 62A) is received in the first mateable portion (32, 32A-G), and the first key (42, 42A) is received in the second key (744, 744A), wherein the first key (42, 42A) is located outward of the first mateable portion (32, 32A-G) in the perpendicular plane; and the second key (744, 744A) is formed in the housing (70, 70A, 70E, 70G) and extends from the projecting end.