P1

From Simplex

PRINCIPLE 1: An opening bid, overcall or response of One of a Suit always guarantees five or more cards and 8-16 HCP, except when responding to a double. The hand must also satisfy the Rule of 18 — i.e. total HCP plus the combined length of the two longest suits must equal or exceed 18. (One important corollary is that you cannot respond one of a new suit with a four-card suit.)