A skill rank is a number indicating how much training or experience a character has with a given skill. Skill rank is incorporated into the skill modifier, which in turn improves the chance of success for skill checks with that skill. A character's number of ranks in a skill is based on how many skill points a character has invested in it. It is possible to collect up to five skill points and stack them until needed; they do not have to be spent at level-up. Many skills can be used even if the character has no ranks in them; doing so is called making an untrained skill check.