Also, compared with the king's pawn openings, transpositions between variations are more common and important in the closed games.

Both these 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.

Both of these moves lead to an immense forest of variations that can require a great deal of opening study to play well.

The Symmetrical Defense (2...c5) is the most direct challenge to Queen's Gambit theory—Can Black equalize by simply copying White's moves?

The Baltic Defense (2...Bf5) takes the most direct solution to solving the problem of Black's queen bishop by developing it on the second move.