This diagram shows the position after the moves 1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.Nf3 Bg7. Both sides have made four developing moves : two Pawns and two minor pieces. Although both sides have made the same number of developing moves, the positions are not at all the same.
White has moved the two center Pawns (both two ranks forward) to their best squares and the two Knights to their best squares. Many players consider this an optimal development for White after four moves.
Black has moved two Pawns (both one rank forward), a Knight to its best square, and has fianchettoed a Bishop. Black is now ready to castle.
Are you surprised that we consider castling a developing move? After all, castling doesn't develop the King. It moves the King into safety. Of course that's right, but in another sense the castling move develops your game, if not your pieces. It develops your game because at some point you need to get your King out of the center and into the corner. You also need to move your Rooks into action on a center file. Castling clears the center files so you can place a Rook there.
Why don't we count castling as a developing move for the Rook? You still need to move the castled Rook to a center file, so we count an additional move for the Rook.
[See also Pirc Defense.]
