Arimaa/Setup

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There are 64,864,800 possible setups for each side, although Gold in effect only has half that many, since an east-west reflection amounts to the same setup for the one going first. Players frequently experiment with different setups, but certain guiding principles are usually followed:

  • Elephant centralisation: The elephant usually starts on the d- or e-file; less commonly, on the c- or f-file.
  • Activity: A piece typically does not start directly behind a weaker piece. For example, the elephant should always start in the front row, as should the camel unless it starts directly behind the elephant.
  • Defence: It should not be easy for the opponent to attack along the b- or g-file. Often, the initial setups will see b2, g2, b7, and g7 each occupied by a camel or horse, which might quickly step onto the key square in front.
  • Balance: There should be some strength on each wing.
  • Rabbit protection: Since rabbits can't retreat, they should not begin exposed to easy pulls.

A small minority of the possible setups obey these rules. However, there is still plenty of scope for variation. A setup which buries strong pieces may be a form of handicap.

Elephants tend to stay near the centre until one has a good reason to go to a flank. Therefore, the most basic characteristic of a setup is the placement of the camel and horses. A symmetric setup, with the camel in the centre and a horse on each wing, is the most flexible. However, one planning a quick trap attack might begin with the camel decentralized, seeing that as a way to save time. Silver does this more often than does Gold; if the gold camel began decentralized, the silver horses could both start far away from it, giving Silver an early alignment advantage.

Symmetric setups[edit]

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Each setup is a mirror reflection of itself, except where the elephant and camel are concerned.

The diagram shows two standard symmetric setups. Gold uses the classic 99of9, named for a player who popularized it early on. A 99of9 setup places the elephant and camel on d2 and e2, the horses on b2 and g2, and the dogs and cats on d1, e1, c2, and f2. Rabbits are arranged so that they won't get in the way of friendly pieces; a rabbit advanced in the middle could be a problem, whereas the opponent has less to gain by pulling an a- or h-file rabbit. Eventually, the c- and f-file rabbits might step into the center to block the goal line.

The squares behind the home traps are rarely left empty during the opening, as that could leave a trap highly vulnerable. Cats often start behind the traps, but a dog behind a trap would make that trap safer from attack, since a dog can't be pushed aside by an enemy dog. Cats and dogs are therefore sometimes swapped in the 99of9 setup. Typical gameplay, however, would not see both dogs remain behind the home traps for very long, so it is more common to set up with cats behind the traps and dogs in the back middle. Moreover, the piece behind a trap will most likely be the first to go if the opponent takes over the trap, so it makes sense to have that piece be small. If from the outset one intends to attack on one wing and defend on the other, a cat may start behind one trap and a dog behind the other.

Silver's setup, known as the Fritzlein setup, puts rabbits behind the traps, leaving the cats slightly more active since they may at some point advance along the b- and g-files. If pushed or pulled forward, however, the c7 or f7 rabbit could never step back; this could complicate any home trap control fight for Silver.

Note that the silver elephant does not directly face its gold counterpart. Had Silver placed her elephant on e7, the gold elephant could immediately go to e6 and delay the silver elephant's advance. That would be bad for Silver, unless she had a quick way to attack and force the gold elephant to return home.

MH setups[edit]

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Silver is poised to quickly attack in the west.

At left, Silver uses an MH (camel-horse) setup. Silver plans to advance her horse and camel in the west, so that a gold horse cannot hold b3. Key to Gold's response is his camel, which could move west for defence, or east for a camel-horse counterattack against f6; the gold camel should quickly commit to one or the other, before Silver gets an advantage.

Silver's attacking threat justifies having her elephant directly face the gold elephant. Gold might play 2g Ed2nnnn, but then Silver could play 2s ha7s mb7s cc7w hg7s and 3s ed7wsse. Silver has lost some time with sideways steps, but Gold will lose even more time moving his elephant, which must step around its silver counterpart and go back home to defend c3.

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The setup from this game

At right, Gold plays an asymmetric setup, intending to quickly attack in the west with elephant, camel, and horse. Silver should not respond with a symmetric setup, which would do little to counter Gold's western strength. Here Silver takes an eastern advantage while minimizing Gold's western advantage; the silver camel will slow down Gold's attack. If the d7 horse goes east, Gold might attack c6 with elephant, horse, and dog, while moving his camel east to face the silver horses.

Another idea for Silver (illustrated here) would be to put the elephant in the west and camel and horse in the east, reasoning that Silver will be faster in an attacking race since Silver's attack is further from the elephants. Yet another idea, from this game, is to play an approximate mirror image of Gold's setup, leading to a slow maneuvering opening.

HH setups[edit]

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The setup from this game

At left, Silver has used an HH setup, placing both horses in the west. An EHH attack by Silver would threaten the c3 trap, but might primarily aim to create threats in c6; an EHH attack tends to be less aggressive than an EMH attack. Silver might pull the western gold horse and use the silver horses to block its retreat. This could lead to a horse frame on c6 or a fork between c6 and f6.

When Silver uses an HH setup, the proper response by Gold is an unsettled question. If the gold horse can advance in the west, this may lead to a strong attack on the c6 trap, but forcing the horse past the silver horses can be tricky. The gold camel might move west to help with this, but then the silver camel would be the strongest eastern piece. Alternatively, Gold might attack the f6 trap; the silver camel may have trouble holding g6 with no silver horse nearby. A silver dog is placed on f7 to keep that square as secure as possible; if Gold took f7, the silver elephant would probably have to come east to prevent a cleanup.

As with MH setups, HH setups are less popular for Gold. If both gold horses start on the same wing, Silver can gain time by setting up with the camel already facing them. Silver might also avoid having a horse directly face the gold camel.

Statistics[edit]

Percentage popularity of camel and horse distributions for Gold
Silver responses to a symmetric gold setup
Average number of forward rabbits

These statistics are based on games between two humans rated over 1800. Setups where one wing has no horse or camel are rare and excluded here. Note that in this classification, 'old-style' EHH setups (e.g. with b- and d-file horses) are counted as central horse setups.