When scouts talk about #1's or 3/4 etc, they are usually using a grading scale based on stuff. A true ace - Kershaw, Santana - is pretty rare. That pitcher should have 2 plus pitches ie above avg (60 on the scouting scale). But, besides just a strong pitch, they also need advanced command and control of the zone. Santana had that perfect change up put he also had a very good fastball and his slider was at least avg if not better. He pounded the zone and his k/bb rate was very good. A #2 starter is also pretty rare. They should also have two advanced pitches but maybe a little less control/command. Sickels pointed to guys like Cain, Greinke and Bumgarner as current #2's. A #3 is a really good pitcher. He has one plus pitch (60 on the scale) and two avg pitches to go with it. The Jorge write up seems to suggest that he might develop one 60 pitch (breaking ball) and that his fastball seems to be at least avg. If he can develop his change up to avg, he's a #3 type. If things fall below that, he's a #4 or maybe a bullpen arm. Defining 1-2-3-4-5 Starters - Minor League Ball And it should also be pointed out that this isn't a perfect thing - it's a tool for scouting. Not results. Brad Radke had a better career than Josh Beckett despite Beckett usually being considered a #1 or 2 type pitcher and Radke being the definition of #3. But that article should give you a pretty good idea of what scouting services (BA, minorleague ball, Klaw, BP etc) are thinking when they say a pitcher is a backend guy or future ace.