D'ni numerals are a fictitious base-25 counting system used by the fictional D'ni race and seen in the two computer games Riven and Uru: Ages Beyond Myst, and in Cyan's Myst novels.
Warning: Spoilers follow
It is suggested that you do not read this article or look at the accompanying picture until you have finished the game Riven. A significant part of the Riven endgame is locked until the player has deduced the meaning of the symbols as numerals, and their values.
The numerals also make a cameo appearance in Myst 3: Exile. The meanings of numerals one through eight are presented much more straightforwardly in the Relto Age in Uru: Ages Beyond Myst.
The image to the right from Garternay.com gives you a basic idea of this system. The numerals 1, 2, 3, and 4 are rotated 90 degrees to yield the numerals 5, 10, 15, and 20 respectively. (This principle of Fives misleads many to believe that D'ni numerals are base-5.) These eight numerals are combined to yield numerals for all the numbers zero through twenty-four.
Two additional numerals, a 25 (an 'X' with the square box) and a "cyclical zero" (a forward slash with the square box), are commonly used for special purposes.
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