American basketball leagues at all levels traditionally use single and double digits between 0 and 5 (i.e. 0, 00, 1–5, 10–15, 20–25, 30–35, 40–45, and 50–55). The NCAA and most amateur competitions mandate that only these numbers be used. This eases non-verbal communication between referees, who use fingers to denote a player's number, and the official scorer. In college basketball, single-digit players' numbers are officially recorded as having a leading zero. Teams can have either a "0" or "00", but they cannot have both.[14] Mathematically, these numbers can be considered as senary rather than decimal numbers, since the senary number system uses the same six digits.