Drop chess changes the meaning of capture. Captured material can become future board material through a reserve or pocket.
Crazyhouse and Bughouse share this reserve logic but differ critically in where the captured piece goes:
Crazyhouse → your pocket Bughouse → your partner's pocket
That one rule delta changes strategy, state representation and team coordination.
Pocket / Reserve
A pocket/reserve is off-board material available to be reintroduced by a drop. Pocket inventory is part of the game state and cannot be reconstructed from board occupancy alone.
Drop as a Move
A drop spends a turn placing an available reserve piece onto a legal empty square instead of moving a board piece.
Empty-Square Requirement
In standard Crazyhouse/Bughouse-style drops, the destination must be empty. A drop is placement, not a capture onto an occupied square.
Pawn-Drop Restrictions
In the current Lichess Crazyhouse profile, pawns cannot be dropped on the first or eighth rank. Other drop games may have different restrictions.
Captured-Piece Reuse
The defining family mechanic is reuse of captured material. Exact ownership, transformation and destination pocket depend on the implementation.
Promoted-Piece Identity
A variant must specify whether a promoted pawn remains its promoted type after capture or reverts to pawn identity in reserve.
Drop Notation
Current Lichess Crazyhouse writes drops with @: piece letter (blank for pawn) + @ + destination, for example N@e5 or @d5.
Pocket State in Data
A complete position encoding must include reserve counts/types, and where relevant original/promoted identity. Ordinary board-only FEN is insufficient.
Crazyhouse Core Rules
Crazyhouse keeps standard chess movement/checkmate but adds personal pockets: a captured enemy piece changes ownership and becomes available for the capturer to drop later.
Captured Piece Changes Ownership
In the current Lichess profile, a captured piece reverses colour and enters the capturing player's pocket.
Own Pocket
Crazyhouse sends captured material to your own pocket, unlike Bughouse where captured material is transferred to a partner.
Move or Drop
On each Crazyhouse turn, choose either a normal legal board move or one legal drop from the pocket.
Drops Can Block Check
Because drops are legal moves, a defender can sometimes answer a line check by inserting a pocket piece on an intermediate square.
Drop Checkmate
Current Lichess Crazyhouse permits drops that immediately checkmate, including pawn-drop mate; this differs from shogi's pawn-drop-mate restriction.
Captured Promoted Pawn Returns as Pawn — Implementation Profile
Current Lichess Crazyhouse returns a captured promoted pawn to reserve as a pawn, not as its promoted piece. This must be encoded as identity state.
Crazyhouse Platform Differences
Crazyhouse communities/platforms can differ on draw rules, repetition, promoted identity and notation. A platform profile must accompany any edge-case rule claim.
Pocket Awareness
In drop chess, Pocket Awareness must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Drop-Square Control
In drop chess, Drop-Square Control must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
King-Safety Geometry
In drop chess, King-Safety Geometry must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Knight Drops
In drop chess, Knight Drops must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Pawn Drops
In drop chess, Pawn Drops must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Initiative
In drop chess, Initiative must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Material Values Change
In drop chess, Material Values Change must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Sacrificial Material Recycling
In drop chess, Sacrificial Material Recycling must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Crazyhouse Opening / Endgame Differences
In drop chess, Crazyhouse Opening / Endgame Differences must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Four Players / Two Teams / Two Boards
Bughouse is normally a four-player, two-team, two-board game with teammates sitting on opposite colours.
Teammates on Opposite Colours
A standard Bughouse team pairs one White player on one board with one Black player on the other, balancing colour across the team.
Captured Piece Goes to Partner
In Bughouse, a captured opponent piece normally becomes reserve material for the capturer's teammate on the other board.
Partner Pocket
The partner's pocket therefore depends on events on the other board; the two boards form one coupled game state.
Drop
In drop chess, Drop must be evaluated with both board and reserve state. A move can change immediate geometry and future drop resources, so ordinary material accounting is incomplete.
Communication
Communication can be a formal team resource, a prohibited aid or an informal social behavior; encode the rule rather than assume.
Piece Requests
Partners may request useful pieces in implementations that allow communication. Such requests affect team strategy but do not create the piece; it must still be captured legally on the partner board.
Piece Flow
Piece flow is the cross-board economy of captures and drops. A tactically favorable capture can be strategically bad if it hands the opposing team a decisive drop piece.
Clock Coupling
Bughouse clocks are strategically coupled because one board can wait for material or for the partner board to finish. Time values depend on both positions.
Sitting / Stalling
Sitting or stalling means intentionally not moving while waiting for the partner board to change the reserve situation. Whether/how it is permitted or strategically useful is implementation-specific.
Partner Sacrifice
A team may sacrifice material on one board to deliver a needed pocket piece to the other. Evaluate the exchange at team level, not one-board material level.
Platform / Club Rule Differences
Bughouse has substantial local variation. Always profile mate timing, draw result, promotion identity, communication and clock handling for the specific club/platform.
Mate / Draw / Promotion Rule Variants
Implementations can differ over whether a mate ends the whole match immediately, how double-mate/timing is resolved, how draws score and whether promoted pieces revert on capture. Never assume one universal profile.
Crazyhouse vs Bughouse
Both use drops. Crazyhouse recycles captured material to the capturer on the same board; Bughouse transfers it to the partner on the other board.
Drop-Chess Practice Set
A strong understanding includes tracking pockets, legal drops, promoted identity, check-blocking drops, changed material values and—Bughouse only—cross-board piece/clock flow.
Material Has Two Locations
In drop chess, material evaluation must distinguish board force from reserve force. A knight in a pocket is not simply an off-board knight worth the same as an orthodox knight: its ability to appear on many empty squares in one move gives it a different tactical profile. Conversely, a queen on the board may be a liability if its capture supplies a decisive checking drop.
A useful evaluation therefore asks:
- what material exists on the board?
- what material exists in each pocket?
- which drop squares are tactically available?
- what pieces can be captured next and recycled?
King Safety Is Drop-Square Geometry
Orthodox king safety focuses on lines, pawn shields and nearby attackers. Crazyhouse adds empty-square access. A hole next to the king can become an immediate knight, pawn, bishop, rook or queen attack depending on the pocket. Defending a square can be insufficient if the dropped piece arrives with check and cannot be captured.
Train by mapping legal checking drops before calculating conventional checks.
Bughouse Requires Global Decision-Making
A bughouse move can be strategically judged only with both boards visible. Before sacrificing material, ask what the capture gives your partner, whether the requested piece arrives in time, and how both clocks affect the exchange. A mating attack can be correct to delay if your partner is about to supply a decisive piece; a winning local line can be wrong if it feeds the opponent's other board.
This is why bughouse databases need event-order, pocket and clock state in addition to board diagrams.