Bridge bidding systems that incorporate a strong 2 clubs opening bid include modern Standard American, standard Acol, 2/1 game forcing and many others. In most natural bridge bidding systems, the opening bid of 2 is used exclusively for hands too strong for an opening bid at the one-level. Typically, the bid is reserved for hands that are almost strong enough to bid to the game level on their own power, or even stronger. The exact requirements for the bid vary considerably depending upon the system used and partnership agreement. In most early bidding systems, opening bids of two of a suit signified a very strong hand and were referred to as strong two bids. However, pioneer bridge inventors like Pierre Albarran and David Burnstine saw that the frequency of such bids is fairly low, and that a 2 bid can be used for all strong hands, leaving other two-level opening bids for other purposes (for example, weak two bids). Bid requirements The strength requirements for the 2 bid differ slightly in different systems. In all cases they show a hand which is close to game forcing. For balanced hands, a 2 bid shows 22 or more points in Standard American (Yellow Card), and 23 or more points in standard Acol. For unbalanced hands, the typical strength is about 9 or more playing tricks, or 3 losers or less if using Losing-Trick Count. Responses Natural responses With "natural" responses, which is the most commonly used treatment, the 2 bid is artificial and very weak (up to 6 HCP). All other bids are natural and positive. Waiting and positive 2 Some players alter or reverse the order of natural responses in order to preserve bidding space and allow for more accuracy in later bidding. There are several treatments in circulation: Waiting 2 – a response of 2 is a relay asking the opening bidder to further describe the strong hand. This bid does not limit the responder's hand in any way. Some players combine this response with each of the following. Natural 2 and 2, typically showing at least a game-going hand (4 or more HCP) with at least a five-card major. Some players go as far to require a 6-card suit with 2 top honors for a suit response. Weak 2. The response of 2 shows a very bad hand (0-3 HCP), making the 2 relay a game-forcing bid. Positive 2