The Philidor Defense is an opening played by black where Black responds on move no.2 with d6 to protect his pawn on e5. After the moves - 1. e4 e5 2. Nf3. Today, the Philidor is known as a solid but passive choice for Black, and is seldom seen in top-level play except as an alternative to the heavily analysed openings that can ensue after the normal 2...Nc6.
Here is a list of the 4 most popular continuations in the Philidor Defense:
a) Exchange Variation
b) Bird's Gambit
c) Exchange Variation with Qxd4
d) Bishop Variation with Bc4
Exchange - Qxd4 Variation