Each piece has a slot (the groove) cut all along one edge, and small outcrop (the tongue) on the opposite edge. Two or more pieces thus fit together.
... - 3rd century - 4th century Decades: 190s 200s 210s 220s 230s - 240s - 250s 260s 270s 280s 290s Years: 237 238 239 240 241 - 242 - 243 244 245 246 ...