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.