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.[1] [2] However, pioneer bridge inventors like Pierre Albarran and David Burnstine[3] 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).