Clearing the King's side for castling, and posting the Bishop ready for
attacking the present weakest point in Black's position--viz., the K B P;
weakest because defended by the King only.
... 3. B to B 4
No immediate danger being apprehended, Black brings a piece into play.
These three moves on either side constitute the opening called the Giuoco
Piano, whatever be the moves that may follow.
4. P to Q 3 4. P to Q 3
5. B to Q 3 5. B to Kt 3
Better than 5. ... B takes B, because in that case White would obtain an
"open Bishop's file," and double his Rooks upon it after castling; and the
"double pawn," which is in the majority of positions a weakness, is strong
in the centre.