I noticed that jersey numbers in NCAA basketball are confined between 0-5 (for both digits). There seem to be no jerseys with numbers between 6-9 (for both digits). I can imagine the practice of retiring numbers becoming a problem if this is the case.
Why do NCAA basketball jerseys only use numbers that are 0-5, 10-15, 20-25, 30-35, 40-45, 50-55, or am I wrong? Is there a rule that defines such? If so, why was this rule implemented?
There is a rule about this. From Wikipedia:
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.
Basically, it eases the communication between the refs and the scorers; since you only have five fingers per hand, refs can only denote numbers one through five non-verbally. After a foul, if you watch the ref you'll notice that they show the scorers which player it was by showing their jersey number with their hands.
Edit:
This can be found here at the bottom of the grid. In the NCAA pdf on basketball rules, Rule 3, Article 5.b.2 the useable numbers are shown (this is on page 41 in the link):
The following numbers are legal: 0, 1, 2, 3, 4, 5, 00, 10, 11, 12, 13, 14, 15, 20, 21, 22, 23, 24, 25, 30, 31, 32, 33, 34, 35, 40, 41, 42, 43, 44, 45, 50, 51, 52, 53, 54, and 55. Team rosters can include 0 or 00, but not both.
