Perhaps the most fitting name of all the variants. This one is a very aggressive game, full of all-or-nothing attacks and sudden counter-strikes. Normal chess, with extra concept of piece drop.
1. The rules
Crazyhouse is played like standard chess, except that pieces you capture become yours to use as you wish on a future turn (and vice versa for your opponent). Instead of your normal move, you can "drop" a captured piece on any empty square on the board, including checking (or checkmating) the King. Pawns cannot be dropped on the 1st or 8th rank, and if a promoted pawn is captured, it reverts back to a pawn (so be sure you know which Queen you are hunting!).
The positions which appear on the board during CrazyHouse game are legal chess positions, only moves are different)
Here on SchemingMind, view captured material via the "Material" tab. Here you can click on any piece of your opponent's colour, and then click on the square you wish to place it. In game notation, @ symbol is used for piece drops, for instance N@f3 means dropping knight on f3.
2. Tips and Strategy
Crazyhouse is all about initiative.
2.1. General Strategy
Since captured pieces keep coming back into play, there are no endgames in crazyhouse, and games are normally won by mating attacks. In typical crazyhouse game both sides develop, exchange some pieces, try to create hooks for an attack in the enemy camp, finally direct attack is unleashed by one of the players, and either succeeds, or faces counter-strike.
Be aware of open, weakly-defended squares in your position and your opponent’s, especially around the king. These squares make ideal targets to drop pieces. For that reason, it is best not to move too many pawns, as this will create holes on your second and third ranks, which can later be exploited. A common strategy is to use a chain of pieces to infiltrate the opponent’s territory. For example, a connected chain of pawns can exploit holes in the opponent’s pawn structure, or a knight on the fifth rank can support pieces dropped onto the sixth and seventh ranks.
It often happens, that both players have attacking chances against the king, and the game becomes a race to see which attack can succeed first. In these situations, try to plan your attack so that every move is a check, that way the opponent never has a free move. If you are on the defensive, and the opponent plays a move that is not check, before you play a defensive move, or make an automatic recapture, look to see if you can launch a forcing attack of your own.
Material balance is relatively unimportant in crazyhouse (compared to standard chess), and it often pays off to sacrifice material in order to expose the opponent’s king. On the other hand, one cannot be careless with material, because pieces captured by the opponent can come back to hurt you later.
It is hard to estimate the relative value of different pieces in crazyhouse, since the specific situation often dictates what piece is most useful. In general, the ranking of pieces in hand (i.e. available to drop), from strongest to weakest, is queen > knight > rook > bishop. The pawn value depends on count - one pawn can be valued similarly to bishop, but four pawns at hand are often worth a queen.
Knights are very strong, because it is impossible to defend against a knight check with a dropped piece. The high value of pawns is due to the ability to place them on the seventh rank, threatening to promote. Also, because each piece type has unique tactical uses, it is usually best to have a variety of pieces in hand (for example pawn plus rook plus knight is generally better than three pawns, or three rooks, or three knights).
Non-standard material configurations are quite frequent. It can happen, that four white knights fight against four darksquare black bishops, or two queens face three rooks.
While many tactical patterns known from standard chess apply also in crazyhouse, the concept of piece drop changes a lot.
First, it is important to distinguish contact check from remote check. Contact check is a check, which can not be refuted with defensive piece drop - given from the square next to the king, or given by knight (double check also can serve the same purpose). Contact checks are usually preferred - and stronger - than remote ones. Mates given by remote rook are almost impossible. Mates given by knight are very frequent.
While calculating and organising an attack, the pieces kept at hand (by both attacker, and defender) are often more important than pieces already present on the board. In fact, it often happens that the whole attack is developed by dropped pieces only.
Contrary to the standard chess, piece exchanges do not help the defender. Exchanged pieces go back to the attacker hand, and can be freely dropped on any suitable square.
One common mating motif is to first give check with a knight, and then on the next move drop a piece (usually a queen or a rook) on the square just vacated by the opponent’s king.
Beware of knight forks, if opponent have knight at hand (or can easily capture one), try to cover all the squares where fork (especially major fork) could happen. One frequent mistake - white castles and while regrouping his forces, puts the queen to g3. Suddenly black knight is dropped on e2, game over.
Watch out also the smothered mates (or try to construct them, they are beautiful).
3. Example games
Good Old Fashioned Crazyhouse - interesting game showing many crazyhouse-specific maneouvres (sacrifice on f2, attack on weak g7, drops inside enemy camp),
Drag the king to death - characteristical middlegame attack, keeping initiative and preparing mate net via the sequence of close checks (also note that earlier white used 3 bishops, and black 3 knights)
Push a king too far - Black's early attack is strong and forces White to run his King, but after several key captures White turns the tables and counter attacks with viscous force.
F7 sacrifice - f7 sacrifice, supported by dropped pawn chain, and additional sac to totally expose black king.
Attacking pawn - such opening would fail in standard chess, but worked perfectly here.
Tempi or material - white outplays black in the opening, but tries to consume the material too early. With queens exchanged, counterattack is unstoppable.
See also opening traps games selected by Space
4. Addtional note about bughouse
For many people crazyhouse is a single-man version of bughouse. Bughouse is a game played on two boards between two pairs, the rules are the same as those of crazyhouse, but the pieces you take can be used by your partner, not by you (this make it suitable for play live - there is no need to change piece colour, you just pass the taken piece to your partner). The side winning on any board (by checkmate or on time) wins the whole game.