What is the Philidor position?
It is a rook ending with one pawn. One side has a king, a rook and a pawn. The other side has only a king and a rook, and is trying to draw.
The defender needs two things. The king stands in front of the enemy pawn. The rook stands on the defender's third rank, which is the rank the pawn has to cross to reach the sixth.
When both are true, the defence holds. The attacker cannot bring the king forward, because the rook covers the whole rank in front of it.
Which rank does the rook sit on?
Ranks are the rows across the board, numbered 1 to 8 from White's side.
The rule is easier to say from the defender's own point of view. Your own back row is the first. Count to the third row, which for Black is the board's sixth rank. That is where the rook sits on a6 in the diagram above.
Why that rank? Because a pawn on the fifth rank needs its king on the sixth to make progress. If your rook owns the sixth rank, the enemy king can never stand there.
What do you do when the pawn steps forward?
Sooner or later the attacker plays the pawn to the sixth rank. That takes the rank away from your rook.
The moment that happens, drop your rook to your own back rank. In the study Black plays 1...Ra1.
From the back rank the rook checks up the file. The pawn on e6 has taken away the shelter its own king needed, so the white king has no safe square anywhere on the board.
Why does the rook go all the way back?
Distance is the point. A rook checking from three or four rows away cannot be attacked by the enemy king.
If the rook checks from close by, the king simply walks up to it. The rook has to move again, and while it is running the pawn goes forward.
So the rule is short. Far away, and behind.
What goes wrong if you sit still?
The second chapter of the study shows the common loss. Black keeps the rook near the pawn with 1...Ra5 and waits.
White answers 2.Kf6. Now the white king is in front of its own pawn, the black rook is within reach, and the pawn goes forward with check.
Waiting is not a plan in this ending. The rook must be doing one of two jobs: holding the sixth rank, or checking from the back rank.
Why the sixth rank matters so much
The third chapter shows the position Black must never allow. The white king has reached f6 while the pawn is still on e5.
From there White plays a check, pushes the black king aside, and brings the king to f7 or d7. The pawn then walks in.
That is the reason the rook sat on the sixth rank in the first place. It was not waiting. It was keeping the white king out.