Chess Opening Theory/1. e4/1...e5/2. Nf3/2...d6/3. d4/3...Nd7/4. Bc4

White develops their bishop to an active square, where it targets Black's weak f7 pawn.

This is a sharp position where several ordinary seeming moves become blunders, as White can take advantage of Black's comparatively passive play to extract material concessions.

4...Be7?, developing the bishop to its only available square, loses the pawn 5. dxe5, because any attempt to recapture it result in Black tactically losing a minor piece due to pressure on f7 (5...dxe5 6. Qd5! or 5...Nxe5 6. Nxe5 dxe5 7. Qh5).

4...Ngf6? develops the knight and seems like an improvement, as it controls the d5 and h5 squares preventing White's queen from infiltrating. But after 5. dxe5 Black still cannot recover the pawn:

• 5...dxe5 allows 6. Ng5! (without ...Be7 g5 is unguarded) and Black has to give up at least a piece to prevent Bxf7 Ke7 Ne7!, trapping the queen.
• 5...Nxe5 is better, but allows White to seize the f7 pawn and trade queens (5...Nxe5 6. Nxe5 dxe5 7. Bxf7+ Ke7 [to keep guarding the queen] 8. Qxd8+ Kxd8).

The danger after both 4...Be7? and 4...Ngf6? comes from the threat of dxe5, so one option for Black is to play 4...exd4 first, eliminating the pawn. Then Black is free to develop their minor pieces.

The most common line is 4...c6. This adds control to d5 (so if 5. dxe5 dxe5 there is no Qd5) while opening a line for Black's queen to develop.

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Wikipedia has related information at Philidor Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

• Schiller, Eric (2002). Standard Chess Openings. ISBN 1-58042-048-6.

• Miodrag Perunovic (2019). Chess For Beginners: Anand & Topalov easily get an edge vs Philidor Hanham variation.
• Nicolas Skettos (2022). How To Beat The Philidor Defense - Hanham Variation - Black Lion - Chess Opening Traps.
• Sean Lei (2020). My Precious "Philidor". Philidor and Lion Defence Chess.