Many species of the Celestial Realm resemble humans, but for a pattern of bumps and ridges on their foreheads. Each such pattern is distinctive to a particular species.
It has been speculated that the patterns are letters in the language of the gods or the Great Race, who in this theory are supposed to have created all such species.
Advocates of this theory are generally evasive about how these letters would have been used. Some resort to a mystical interpretation, while others assert that the Celestial Realm was used as a gigantic equivalent to a crossword or Scrabble board, with each species counting as a single filled-in letter or tile, and its planet of origin being the square.
The smooth foreheads of humans are said to be analagous to spaces, or to blank tiles.