The following shows the amount of experience a character must earn to gain a level in their character class. All experience earned by multiple-class characters is divided by the number of classes. When a character has earned enough to gain a level for one class but not another, the Show command shows the highest level.
Thus, a character who is a fighter/thief and has earned 1,800 experience points in each class (for a total of 3,600 XP) will display as being 2nd level. This is because they have earned that level as a thief, but not as a fighter. Their fighting abilities are still based on their being a 1st level fighter.