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.
  • Balance: There should be some strength on each wing, so that each home trap has some protection.
  • 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 strong enemy pieces draw them to the outskirts. Unless both sides play a lone elephant opening, an early camel or horse advance could set the tone. The placement of the camel and horses is thus the essential characteristic of a setup. A symmetric setup, with the camel in the middle 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.

As the least exposed key squares, the squares behind the traps are almost always occupied during the opening. If for instance move 2g left c2 empty, Gold might quickly lose a piece in c3. 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. A disadvantage of this is that a dog could get stuck behind a trap, perhaps weakening a hostage position. Moreover, the piece behind a trap is usually the first to go if the opponent takes over the trap, so it makes sense to have that piece be small. It is thus not very common to set up with both dogs directly behind the traps.

Silver uses the Fritzlein setup, placing rabbits behind the traps; this setup leaves 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. With that said, winning a fight at home may not be a priority for one inclined toward aggressive play.

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 might 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, if the gold elephant stays on d6, 3s ed7wsse. Silver loses 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. When Silver uses an MH setup, Gold must either attack first or be prepared to quickly defend. In any event, an elephant should not immediately advance four squares if that will allow for a strong attack by the opponent.

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The setup from this game.

At right, Gold has set up for a quick EMH attack in the west. To slow this down, the silver camel directly faces the gold camel. Silver could place both horses in the east, far away from the gold camel, but prefers to place one horse in the middle so that it can quickly go to either wing. Gold might move his camel east and still get a strong western attack if there is no silver horse in the west; to avoid being pulled toward c3, the silver camel may need a supporting horse.

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.

Placing his horses on the a- and b- files and his elephant on the c-file, Silver intends to create a fairly quick threat in the west. While a full-on EHH trap attack could occur at some point, an HH setup is usually played less aggressively than an MH setup. 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. An EMH attack against c6 could leave Gold vulnerable in the east, where the silver camel would then be the strongest piece. Gold might do better to attack in the east; the silver camel might 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, no silver piece would be safe in the east until Silver abandoned the western EHH attack.

For Gold, an HH setup is perhaps even riskier than an MH setup. If the gold horses start out side-by-side, the silver camel can begin directly opposite both. Silver might also avoid having a horse directly face the gold camel.

Rusty setups[edit]

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Setups preferred by the top Arimaa bot.


These setups are commonly used by the top-rated Rusty_Zero, a self-taught neural networking bot. The gold setup is designed to initially attack in the east and defend in the west, hence the cat on f2 and the dog on c2. Not wanting a camel gridlock, Rusty typically places his camel on the wing opposite that of the enemy camel. If Rusty plays as Gold, and Silver places his camel directly opposite Rusty's, Rusty will quickly move his camel to the other wing. This is perhaps why Rusty often places his western horse on the a- rather than b-file; it won't have to get out of the way if the camel comes west.

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 each 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.