They start at 2.0 and go up .5 ratings all the way up to 7.0 but for recreational tennis players the top limit for leagues is usually 5.0 with many smaller areas maxing out with leagues for 4.5 level players.
A perennial issue raised by the ratings system is when new players join league tennis they must self rate for their first year before they are given a computer rating. If they have self rated too low and the computer catches them they may be bumped up to the next higher level and their previous match wins are overturned. This is called a disqualification. Once they have gotten their computer rating they are no longer at risk for a disqualification.
Unscrupulous team captains have been known to manage their self rated players rankings so they do not get disqualified but still are able to help the team go on to post season play. For some of the USTA leagues there are Districts, Sectionals and even National tournaments where the winning teams compete to see who is the best at their level.
In my years of USTA league play I've only seen three blatant examples of captains cheating the system:
The first example was when a captain convinced a player to play two levels below his ability. The captain then proceeded to play every match with the player and purposely throw games. The captain went so far as to lose the first three matches the two played together. This caused the new players DNTRP (dynamic rating) to be at the lower end of the ratings scale and ensured that the player wouldn't get disqualified.
The second example was when a captain entered a self rated players name in several matches that he didn't play so that his DNTRP would be low. This was done before the player even stepped out onto a court and then when he played he easily beat his opponents but had already gotten a lower dynamic rating.
The third example was when a teaching pro self rated at a lower level. The captain played him straight up for his first match and it went as well as could be expected with him easily winning. For the players second match the captain brought 2 potential partners, one who was top of the level if they would play a tougher team and one who was at the bottom of the ratings level. He partnered with the lower level player and was clearly throwing games to keep the score competitive.
Whenever players encounter this blatant cheating it takes away some of the joy of the game. To address this I would recommend that self rated players should not be allowed to play post season until they receive their computer rating.
