Balanced ternary
The most common numbering system in use today is base 10, or decimal. With decimal, there are ten digits. these are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.   In ternary only 0, 1 and 2 are used. Balanced ternary is similar, but instead of using 0, 1 and 2,  the numbers -1, 0 and 1 are used. The letter T is used to mean  -1.