This is the short version, written for players who already know how a King, Queen, Rook, Bishop, Knight, and Pawn move.
Decachess plays on a wider board and adds one new piece, plus four changes to rules you already know.
Everything else — check, checkmate, stalemate, how captures work — is exactly as you'd expect.
The Board
10×10 instead of 8×8 — files a through j, ranks 1 through 10. Two extra pawns per side, and one new piece (the Archer) slotted in beside each Rook.
The Archer has a short walk and a separate ranged shot, and never does both in the same turn.
Move: one square in any direction, king-style — but only onto an empty square. It cannot capture by moving into a piece.
Attack: strikes any enemy piece exactly two squares away in a straight line (orthogonal or diagonal). The Archer does not move to do this — it stays put, and the arrow ignores anything standing in between.
Silenced at range 1: if any enemy piece is directly adjacent, the Archer cannot fire that turn. Closing in shuts it down.
Rooks are immune: no arrow can harm a Rook, at any range, from any angle. This is absolute.
Worth remembering mid-game
The Rook immunity is the rule people forget most often — the geometry can look completely valid and the shot still won't be offered, because the target is a Rook specifically.
Castling
Same underlying rule as standard chess — unmoved King and Rook, clear path, King not in/through/into check — but the wider board means more has to be cleared first.
Kingside: King moves 2 squares toward the Archer's side (f→h for White). The Rook jumps to the square beside the King's new position. Requires the Knight, Bishop, and Archer on that side to have already moved.
Queenside: King moves 2 squares the other way (f→d for White). Requires the Archer, Bishop, Knight, and Queen to have already moved — a bigger commitment than kingside.
Castling early is rarely realistic here; it tends to be a mid-game payoff once you've actually developed.
Pawns: The Triple Leap
A pawn's first move can be 1, 2, or 3 squares forward (path must be fully clear). This exists purely because a 10-wide board is a lot of ground to cover — without it, the center takes too long to contest. Every move after the first is a single square, same as always.
En Passant
Works exactly like standard en passant, extended to cover the 3-square leap. The window still lasts exactly one move — miss it and it's gone.
2-square leap: capturable by an enemy pawn already sitting on the landing rank, adjacent file — same as standard chess.
3-square leap: skips two squares instead of one, and each skipped square has its own independent trigger — an enemy pawn on the middle rank catches the nearer square; an enemy pawn on the landing rank catches the farther one.
Promotion: Graveyard Reinforcement
The single biggest departure from standard chess. There is no automatic queening. A pawn reaching the far rank can only come back as a piece your opponent has already captured from you.
Reaching the back rank lets you choose any piece from your own graveyard — pieces of yours already captured. Any type is eligible, pawns included.
If your graveyard is empty when the pawn arrives, it doesn't promote — it freezes on that square. Still fully capturable, just immobile, until you lose something.
The moment you lose a piece afterward, a frozen pawn automatically offers you a choice of what to bring back. Multiple frozen pawns resolve first-arrived-first-served.
However it resolves, the returned piece cannot move or attack on the turn it appears — it has to wait one full turn.
Why it matters
This turns "push the pawn to the back rank" from a free bonus into a real decision. Arrive with nothing to reinforce with, and you may be donating a piece — or setting a trap, if something's guarding it from behind.
The Controls
A quick reference for the control panel — a couple of these don't behave quite how the button label alone would suggest.
Game Mode / Play As
Two dropdowns controlling who plays which side, and whether the opponent is the built-in AI or a second human. Changing either one immediately restarts the game — neither is a live mid-game toggle.
AI Level
Novice, Veteran, or Master. This one is safe to change mid-game — the new setting applies to the AI's very next move, without resetting the board.
Manual Move Entry
Type a move directly instead of clicking — format is fromSquare-toSquare, e.g. e2-e5 or b1-b3. File letters aren't case-sensitive.
Undo Move
Steps back up to 4 plies at once — your last move and your opponent's reply, twice over — or as far back as the game actually goes if it's shorter than that. It is not a single-step undo.
Copy Moves / Paste Moves
The portable way to save or restore a game as plain text. Copy Moves puts the full move list on your system clipboard. Paste Moves reads directly from your system clipboard — there's no text box to paste into, so copy a move list first. Pasting always replaces the current game entirely.
Save State / Load State
A separate, simpler mechanism — stores one exact snapshot locally in your browser (a single slot; saving again overwrites the previous save). Instant, but only lives on this device.
Reset Board
Immediately restarts to a fresh game. No confirmation prompt.
Quick Reference
The Archer
Moves 1 square to empty only. Shoots exactly 2 squares in a straight line, no blocking. Silenced if an enemy is adjacent. Cannot ever harm a Rook.
Castling
Same safety rules as standard chess. Kingside needs 3 pieces cleared; queenside needs 4.
Pawns
First move only: 1, 2, or 3 squares. Every move after: 1 square.
En Passant
One-move window, same as standard — extended to cover both the 2- and 3-square leap.
Promotion
Resurrect from your own graveyard, your choice. Empty graveyard = pawn freezes until you lose something. Returned piece waits one turn before it can act.
BlackSheepConcepts79@gmail.com