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.
... (+ 1983) January 26 - Ancel Keys, scientist February 1 - S. J. Perelman[?], humorist, author February 4 - MacKinlay Kantor[?], historian (+ 1977) February 5 ...