P2

From Simplex

PRINCIPLE 2: An opening bid, overcall or response of Two of a Suit always guarantees six or more cards and 3-16 HCP, except when responding to a double. (So, for example, you cannot respond to 1NT with 2 of a suit, part from 2♣, unless you have six cards in that suit. And you cannot bid a natural overcall over 1NT with just five cards in the suit.)