Signed-digit Representation
In some systems, while the base is a positive integer, negative digits are allowed. Non-adjacent form is a particular system where the base is b=2. In the balanced ternary system, the base is b=3, and the numerals have the values −1, 0 and +1 (rather than 0, 1 and 2 as in the standard ternary system, or 1, 2 and 3 as in the bijective ternary system).
Read more about this topic: Non-standard Positional Numeral Systems