Stacks in practice

The rule chess never bends is one piece per square. Two Built Deck cards bend it. Pile On stacks one of your own pieces on top of another; Consolidate stacks two of your opponent's. Once a square holds two pieces, the questions start: which one moves, which one gets taken, which one gives check, and whether a stacked piece is safe or stuck.

The Study Hall lesson on stacks shows the mechanics on a board. This is about what to do with them.

How a stack behaves

A stack has a top piece and a buried piece, and the top piece is the square. It's the one that moves, the one that captures, the one that attacks and gives check, and the one an enemy capture takes. The buried piece does none of those things. It can't be captured while something stands on it, and it can't attack anything either. It's inert until the top piece leaves.

Stacks only form through the two cards. No move ever stacks pieces; moving onto a square with your own piece on it is as illegal as it ever was.

A stack unstacks one piece at a time. When the top piece moves off, by its own movement pattern, the buried piece is a normal piece on a normal square again. To use a buried piece, you first move whatever is on top of it.

A capture takes the top piece and leaves the rest standing. The square isn't emptied, and the capturing piece stays put instead of moving onto it. A stack is a damage sponge: it takes two captures to clear it, and the second one has to wait for the first.

A king may stand on top of a stack and is checked and moves normally there. A king is never buried.

Both players see the whole stack. There's nothing hidden about what's underneath.

Pile On: a shield you can't lose

Pile On costs three Braincells and is a defense card as much as a modification. You choose two of your own pieces, and one goes on top of the other. From then on the buried piece can't be captured. Not by a queen, not by a knight, not by anything, until the piece on top of it is gone.

That makes it a shield with no expiry. Shield protects until after your next turn. Aegis protects for two full turns. Guardian Ward protects until the piece moves. A buried piece is protected until your opponent spends a capture on the top piece, and that capture costs them a move and exposes the capturing piece to recapture.

The uses follow from that.

Bury the piece that's in trouble. Your queen is attacked and has no good square. Stack a pawn on top of it, and the queen is gone from the board as a target. Your opponent can take the pawn, but then you recapture or the queen moves out, and all they've won is a pawn.

Bury the piece you'll need later. A rook you want for the endgame can sit under a pawn through the middlegame, untouchable. When the position opens up, move the pawn, and the rook is where you left it.

Put the sacrifice on top. The piece on top is the one that gets taken, so choose it to be the one you mind least. A pawn on top of a queen is a pawn's worth of insurance on a queen.

Pile a defender onto a key square. Two pieces on one square means the square stays yours even after one is taken. A knight stacked on a bishop on a central square holds that square through two captures, and the second capture has to come from a piece that's just taken on it.

Pile On: the costs

The piece underneath does nothing. A buried queen doesn't attack, doesn't defend, doesn't give check. If you bury your queen to save it, you've also taken your queen out of the game until you move the piece above it. That costs a move later, and in the meantime the rest of your army is playing a piece down.

The top piece is stuck with a job. If it moves, the stack is over. So the piece on top is pinned by your own plan; you can move it, but only if you've decided the buried piece can come out.

A frozen top piece freezes the stack. Freeze on the pawn above your queen means neither can move until the pawn thaws: the pawn can't move, and the queen can't move out from under it. A 1 Turn Freeze is the cheapest way to answer a Pile On, and your opponent will know it.

Three Braincells is real money. A Pile On that saves a pawn is a bad trade. One that saves a queen, or holds a square the whole game turns on, is a good one.

Consolidate: two turns to undo

Consolidate costs three and is an attack card. You pick two of your opponent's knights, bishops or rooks of the same type, and pile one onto the other. Two knights become one square with a knight on it and a knight underneath.

What it does to them is subtle. They haven't lost a piece. They've lost the use of one: the buried knight can't move or attack until the top one moves off, and that costs them a move they'd rather spend elsewhere. The card text says it plainly: they now need two turns to bring both back into play.

What it does to the board is less subtle. Two rooks that were on two files are now on one. A bishop pair that covered both colors is one bishop covering one. Whichever squares the buried piece was guarding are unguarded now, and they stay unguarded until your opponent pays the move to unstack.

That's the moment to use. Consolidate their rooks, then attack the file the buried rook was on. Consolidate their knights, then put a piece on the square the buried knight was watching. The card buys you one tempo of their absence, and you want a plan ready for it.

The best time to play it is when one of the pair is doing something important. Stacking the knight that's defending their king under the knight on the other wing takes the defender off the board for a turn and gives them a choice between unstacking and defending some other way.

Capturing into a stack

When you capture the top of an enemy stack, your capturing piece stays where it was, and the buried piece becomes a normal piece again on the square you just took from. Before you take the top, look at what's underneath and ask what it attacks once it's free. Taking a pawn off the top of a queen means a queen is now on the board, un-buried and probably not pleased.

Sometimes that's fine: the queen is where it was and your capture was a free pawn. Sometimes the buried piece springs out onto exactly the square you needed, and you've unstacked their shelter for them.

The same goes the other way. Your opponent takes the top of your stack, and the piece beneath is suddenly active again. If you buried a rook under a pawn on an open file, their capture of the pawn hands you a rook on an open file with a target in front of it. Stack so that the unstacking hurts them.

Stacks and the other cards

Freeze on the top piece locks the stack, as above. A shield on the top piece does what a shield does: the top can't be captured either, and for as long as it lasts the whole stack is out of reach. Each piece in a stack keeps its own marks, so a frozen piece buried under a free one is still frozen when it comes out.

When a stack is a trap

Stack a piece under another when the buried piece is the one you want to keep, the top piece is one you can spare, and you have a plan for when it comes out.

Don't stack when the position needs every piece working. A buried piece is a piece you've benched. In a slow, closed position with time to spare, benching a queen under a pawn for ten moves costs nothing. In a sharp position where every tempo counts, it costs the tempo you spent on the card and the tempo you'll spend getting it out.

And don't stack to run away. If the only reason to bury a piece is that it's attacked, ask first whether moving it is cheaper. A move is free. Pile On is three Braincells and a bench.