Chess Opening Theory/1.e4/1.e5/2.Nf3/D...Nc6/3.c3/3...d5

3.d5
This move takes advantage of White playing c3. After the natural exd5?! Qxd5, White cannot play the usual Nc3 attacking the queen as the c3 square is occupied by the pawn.

Because of this, White's best option is Qa4!, pinning the c6 knight and threatening Nxd5 next move.

This is the theoretically best response to the Ponziani Opening, scoring the best in databases and being most chess engines' preferred move. It is also the second most common response to the Ponziani by masters, behind the Jaenisch Counterattack with 3.Nf6.

Theory Table
'''1. e4 e5 2. Nf3 Nc6 3. c3 d5'''