Meaning of tongue-and-groove joint

tongue'-and-groove' joint"

Pronunciation: (tung'un-grv'), [key]
— Carpentry. Carpentry.
  1. a joint between two boards in which a raised area on the edge of one board fits into a corresponding groove in the edge of the other to produce a flush surface.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.