The pawn chain tells you where the game will be played

Every pawn in it is defended except one. Find that one and you have found the plan, usually for the next twenty moves.

Philidor's rule Attack the base, not the head
Where you meet it French, King's Indian, Advance Caro-Kann

A pawn chain is a diagonal row of your own pawns where each one is protected by the one behind it. Only the rearmost pawn, the base, has nothing behind it. That is why the old rule from Philidor still runs the whole subject: attack the base, not the head. The head is defended by a pawn and can be reinforced by another pawn. The base can only ever be defended by pieces.

The second thing a chain does is point. A chain running from d4 up to e5 gives its owner more room on the kingside, and that is where its owner should be playing. Chains lock a position, and a locked position rewards long slow plans instead of tactics, which is why these are the structures where knowing the plan is worth more than calculating.

Both sides get one: the French Advance

Ten moves and the shape of the whole game is fixed.

1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Qb6

White's chain is d4 and e5, its base is d4, and c3 is already propping it up. Black's chain is e6 and d5, its base is e6, sitting safely behind everything. So Black hammers d4: ...c5, ...Nc6 and ...Qb6 all hit the same pawn.

Where to attack, and with what

Attack the base with pawns first, pieces second. Black's ...c5 in the diagram is the model: a pawn is the cheapest attacker there is, and if the base is defended by another pawn you can often attack that one too. White's mirror-image plan is f4 and f5, going after e6, the base of Black's chain, which is harder to reach and takes longer to organise. Both sides know exactly what they are doing and have known since move four.

If the base is genuinely unreachable, you have two other options. Undermine the pawn defending the head instead, so that trading changes which pawn is the base. Or accept the structure and play on the wing your own chain points at, which is the King's Indian answer: White attacks on the queenside where the c4-d5 chain gives him space, Black attacks on the kingside where the e5-d6 chain gives him space, and both sides ignore each other for twenty moves.

Find the base of both chains before you decide anything else.
Attack it with a pawn. Pieces alone rarely shift a defended chain.
Play on the side your chain points at, and defend on the other side.
A blocked chain makes the position slow, so a tempo matters less here than a good square does.
The base changes when pawns come off. Recount after every trade.

Questions people ask

Why do you attack the base of a pawn chain?

Because it is the only pawn in the chain that no other pawn can defend. Attack the head and your opponent simply defends it with the pawn behind it, and you have achieved nothing. Attack the base and every attacker has to be answered by a piece, and if the base eventually falls or has to advance, the pawns in front of it lose their support all at once.

Which side of the board should you attack on?

The side your pawn chain points at, because that is the side where you have more space and your pawns can advance without leaving holes behind them. If your pawns run from c3 to d4 to e5, you have room on the kingside and your attack belongs there. Doing the opposite means attacking into your opponent's space, which almost never works.

Pawn chain sits under Pawn structure in the reference. Nearby: