The Scotch Game is an King-Pawn opening classified as C45 in the ECO code, the opening moves being
- e4 e5
- Nf3 Nc6
- d4 exd4
4... Bc5 is often said to be Black's simplest and most economical answer to the Scotch (check Nigel Davies, Play 1 e4 e5: A complete repertoire for Black in the Open Games).
4... Nf6, followed by 5. Nc3 is also known as the Scotch Four Knights variation.
with 4... Nf6 Black attacks White's centre, which usually takes the oportunity to develop another piece with 5. Nc3. 5 Bd3, 5 e5, 5 f3 are not considered good alternatives, since they usually leave a piece en prise to 5... Nxd4! or 5 Nxe5 and in the last case, it weakens the king's position by opening the diagonals (see John Emms, starting out: the scotch game).