Balanced Ternary

Balanced ternary uses three digits: T (−1), 0, and 1.
Each position represents a power of 3: ...27, 9, 3, 1.
Unlike standard ternary (0,1,2), balanced ternary can represent negative numbers without a sign.
Example: 1T0 = 1×9 + (−1)×3 + 0×1 = 6

It was used in the Soviet Setun computer (1958), the only known balanced ternary computer ever built.