A tongue flange has a raised ring which would fit into the groove installed on a groove flange.
When using tongue and groove flanges, each flange with a trongue surface type should be matched with a flange with a groove surface type.
The T&G surface type has an advantage of self-aligning and providing stronger connection.

Additional Information
Compared with FF and RF, RTJ, T&G and M&F provide stronger connection and better fit. However, they are more expensive than FF and RF, and commercial availabilty can be limited since FF and RF are most commonly used in the market.
In addition, when using T&G and/or M&F, the complexity of designing the piping system may increase since they have to be matched.
Same surface type should be used and it is not recommended to assemble with a different surface type (e.g., assembling RTJ with RF).
When not using the same surface type, a gap may exist in the gasket part and a trustable connection cannot be guaranteed, and thereby a leakage may occur.