P6

From Simplex

PRINCIPLE 6: If Opener/Overcaller rebids his suit on the second round, this shows a suit longer than the five cards originally promised. A simple rebid — e.g. 1♥ then 2♥ — shows a 6-card suit. A jump rebid — e.g. 1♣ then 3♣ — shows a 7-card suit. (NB This principle is not enforced if responder showed a strong hand in the first round — i.e. 2NT, 3x, or 4NT.)