A really odd concept from TSR's D&D game. If you were of the same alignment as another character, you could were able to converse with them without anyone else understanding you. D&D also limited the range of alignments to lawful, neutral and chaotic, so there were only three of these tongues, but it still seemed very odd.

Especially when ``Lawful'', the language, was the only common tongue between your character and, say, an elf from the nearby moon.