I see alot of references to the composition of a frame by what I can only assume is it's base materials. Like when people refer to a frame as a 531, what do those numbers signify? Is there a higharchy similar to the rise in the number itself? Where do Columbia and Reynolds play into this? I know they are manufacturers, but are they just the tube manufacturers, or do they just design the composition of the metals?

Are there any articles and such that can help educate me, or anybody here that can clarify it?