A limit raise is a fit response to an opening of one of a suit. A jump raise of opener's suit typically shows invitational values (10-12 points). Opener should bid game in a major suit with 15 points and pass otherwise. Limit raises were developed because the original natural system for responding to suit openings made it very difficult to describe a hand with invitational values (the only possible response allowed was 1/1 and 2/1 responses), and such hands are far too common for that level of awkwardness.