Stacked Boards

Stacked Boards is played on three full chessboards, one above another: the bottom board (B), the middle board (M) and the top board (T). All three face the same way. White starts on the bottom board and Black on the top board; the middle board starts empty.

How pieces move

Every piece moves exactly as in regular chess on its own board. It can also move to another board:

• Pawn — as it steps forward one square it may also rise or drop one board. On its first move it may step two squares forward and change one or two boards.
• Knight — makes its usual L-jump and lands on that square one or two boards up or down.
• Bishop — may move diagonally up or down: one diagonal square for each board it changes (one or two boards), in a straight line. It can't zig-zag around pieces or keep going sideways afterward.
• Rook — may move straight up or down to the same square on another board, if nothing is in the way.
• Queen — has both the rook's and the bishop's moves between boards.
• King — may move straight up or down one board to the same square. On its very first move only, it may move one board up or down and one square in any direction (including diagonally).

A move to another board can never capture, so the landing square (and any square the piece passes through on the way) must be empty.

Special rules

• First moves change boards. The first time each piece moves, it must change at least one board. Exceptions: castling, and when your king is in check you may capture or block on the same board even with a piece that hasn't moved yet.
• Captures, check and checkmate happen on one board only. A piece attacks only squares on its own board, so check and mate need the attackers on the king's board. Remember the king can escape to another board.
• Castling works as in regular chess on the starting board: king and rook unmoved, the squares between them empty, the king not in check and not crossing an attacked square. King and rook may also castle together one board up or down; their landing squares there must be empty and the king can't land in check.
• Promotion happens on the far row of any board. You choose the new piece.
• No en passant.
• Draws: stalemate, the same position three times, fifty moves without a capture or pawn move, not enough material to mate, or a draw offer the opponent accepts.

Tri-D

Tri-D is played on three 4×4 main levels — White's low level (2), the neutral level (4) and Black's high level (6) — each offset so half of its squares sit over half of the squares of the level below, plus four small 2×2 attack boards on posts at the main levels' corners. Squares are named by file a–f, rank 0–9 and level 1–7, for example c3(4). These rules follow Roth's write-up of the Federation Standard rules.

• Setup: White's knights and bishops start on the low level, with four pawns in front; the king, queen and rooks start on White's two attack boards, with a pawn in front of each. Black mirrors this on the high level.
• Moving by shadow: look down from above and every piece moves as in regular chess across that shadow board. It may finish on any level that has a square there, but it can't move straight up or down without also moving sideways.
• What blocks a move: ignore every level above the higher of the starting and finishing levels; the piece travels along the highest squares left beneath its path (like paint dripping down). Any piece on that route blocks it, and so does a gap with no square at all. If the higher level is a main level with an attack board standing above it, the route may go through the attack board instead — either route will do. Knights jump and are never blocked.
• Captures and check work on any level — unlike Stacked Boards.
• Moving an attack board takes your whole turn. It must be one you own and either empty or carrying a single pawn of yours, and you must still have at least one pawn somewhere. It moves 1 or 2 ranks along its own side to another post, and/or flips to hang beneath its level. Two boards can't share a post the same way up. A carried pawn rides along (and promotes if that puts it on a promotion rank). Capture the last enemy piece on an enemy attack board and the board becomes yours.
• Pawns: two squares on their first move only (a pawn carried by an attack board has moved). En passant is allowed straight after a two-square move, onto the square the pawn passed over. White promotes on rank 8 or 9, Black on rank 1 or 0.
• Castling: king and rook unmoved, and the king not in check before or after. King's side: king and rook swap places. Queen's side, once the queen has left her starting square: the king jumps to the queen's square and the queen's rook to the king's.
• Rook-pawn rule (optional, set on the opening screen): pawns on the a and f files may also step sideways toward the centre (two on their first move) and capture diagonally inward, until they reach files b–e.

To move an attack board: tap Move attack board, tap the board, then choose where it goes from the list (or, with the visualizer on, tap one of the glowing outlines on the 3D view). Tap Cancel board move to back out — except with the visualizer off, where touch-move applies to boards too. The button only appears when one of your boards can move.

Flat view in Tri-D shows each main level with the attack boards on its posts beside it; an attack board's label says its level and whether it stands above or hangs below. A gold or brown frame shows which side owns it.

Playing

Tap one of your pieces to pick it up (it lifts), then tap the square to move it to. When a pawn reaches the far row you choose what it becomes.

3D view: drag on the boards to turn and tilt them; pinch or use the mouse wheel to zoom. The buttons under the boards do the same — Turn, Tilt, Zoom and Reset view. You can turn the boards all the way around and tilt them to look from below. Tap what you see: a piece seen through a clear square is picked rather than the clear square in front of it, and with the visualizer on so is a marked square behind it. A black square blocks the view. If a square you want is still hidden behind another board, tilt or turn the view, or use the flat boards.

Flat boards: the Flat boards button shows the three boards side by side (stacked on a phone), labelled Top, Middle and Bottom. Tap 3D view to go back. You can switch at any time, even with a piece picked up.

The last move is outlined in green on both of its squares. A king in check glows red.

Visualizer and touch-move

Choose the visualizer on the opening screen; it stays set for that game.

• Visualizer on: when you pick up a piece, yellow dots mark every square it can move to on every board or level (a yellow ring marks a capture), and dashed yellow lines lead to the squares on other boards or levels. You can put the piece back by tapping it again, or tap a different piece instead.
• Visualizer off — touch-move: no squares are marked, and once you pick up a piece you must move that piece, as in tournament chess. If a piece you tap has no legal move, you're told so and may choose another.

Options and buttons

On the opening screen you choose the version (Stacked Boards or Tri-D), your colour, how many moves ahead the opponent thinks (1 to 6 — higher is stronger and slower), a time control and the visualizer. There are many more possible moves on three boards than on one, so higher look-ahead settings think much longer than in regular chess; the opponent stops after about fifteen seconds and plays the best move it has found by then.

Take back undoes your last move and the reply. Offer draw is accepted when the opponent isn't ahead. Resign ends the game. NEW GAME goes back to the opening screen. Your game is saved in this browser automatically, so you can come back and resume it.

The move record

In Stacked Boards each square is written with its board letter first: B bottom, M middle, T top — so Me4 is square e4 on the middle board. A move shows the piece (♔ king, ♕ queen, ♖ rook, ♗ bishop, ♘ knight, nothing for a pawn), where it started, a dash for a move or × for a capture, and where it landed: ♘Bg1–Mf3 is a knight jumping from g1 on the bottom board to f3 on the middle board. O-O and O-O-O are castling; an arrow and a letter (O-O→M) mean king and rook castled onto that board. + is check, # is checkmate. The small green number after each move is who is ahead in pawns, from White's side. Copy game record copies the whole game as text.

In Tri-D squares are written file, rank, then level in brackets — ♗c1(2)–e3(4). An attack-board move is written with the board's name — WQL and WKL start as White's queen's-side and king's-side boards, BQL and BKL as Black's — and the post it moves to, with its new level: WQL→b4(1). Levels 1, 3, 5 and 7 are attack-board levels.

The Oracle

The Oracle is your optional advisor. Tap 🔮 to open it and it stays on duty at the start of each of your turns until you close it.

It uses minimax: for every move you could make it imagines each reply, each answer to that reply, and so on, assuming both sides play their best, then scores the move by how that ends. It looks as many moves ahead as the look-ahead setting, within about six seconds in total. It spends that time in two passes: a quick look at every move, then the rest of the time looking deeper at the six most promising. With so many moves on three boards it may still reach fewer moves ahead than the setting; the fine print says how deep it got.

In Tri-D the Oracle also weighs moving an attack board; a recommended board move is outlined purple where the board is now and gold where it would go.

A green tag means the move forces mate; red means it gets mated against best play; amber is its judgement of the position in pawns (bigger is better for you). The Oracle knows the positions already played: a move that would bring the same position up a third time is tagged grey "Draw (repetition)", and a move that lets the opponent reply with a third repetition is tagged "Allows draw (repetition)". A move that repeats a third time is scored as a draw. A move that lets the opponent repeat is scored as a draw only when you're ahead (the opponent would take the draw); when you're behind it keeps its normal score. Either way, when you're ahead these rank below every move that keeps your advantage. It also counts stalemate as a draw anywhere in its look-ahead. It weighs material, where pieces stand (in Stacked Boards, including being on the enemy king's board), room to move, king safety and pawn structure. The best move is shown on the boards — purple outline where the piece starts, gold where it lands, red on anything captured. Tap another line to see it instead. The Oracle only advises you; the opponent plays its own way regardless.

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