The Colle System and Stonewall Attack are classified as Queen's Pawn Games because White plays d4 but not c4. They are also examples of "Systems", rather than specific opening variations. White develops aiming for a particular formation without great concern over how Black chooses to defend. Both systems are popular with club players because they are easy to learn, but are rarely used by professionals because a well-prepared opponent playing Black can equalize fairly easily. The Stonewall is characterized by the White pawn formation on c3, d4, e3, and f4, and can be achieved by several move orders and against many different Black setups. The diagram positions and the move sequences given below are typical.