A beginner’s guide to Omok

What is Omok?

Omok (as its called in Korea) is a simple yet strategically complex game common throughout many parts of Southeast Asia. More commonly referred to as Gomoku in most parts of the world or Renju in Japan (its country of origin), the game is played on a Go board, with the same black and white stones as Go. As stones are not removed from the board as the game progresses, Omok can also be played with pen and pencil on lined paper, though I prefer to play it digitally on the app Dr. Gomoku.

The objective of Omok is simple, create a line of 5 stones in a row (vertically, horizontally, or diagonally) of your own color. Players take turns placing stones on the board, beginning with the black player, until a player connects 5 stones of their own color, the two players agree to a draw, or the board is filled (something which has never come close to happening to me in hundreds of games).

Why Play Omok?

The rules and basic strategy of Omok are incredibly easy to pick up and understand, yet the game offers a tremendous amount of strategic depth. A typical game of Omok takes fifteen minutes or less to complete, making it my de-facto phone game for lengthier bathroom breaks and long car rides. Though not nearly as complex or deep as other classic board games such as Chess or Go, the major advantage Omok has over its competitors is how quick and easy it is to pick up and play. Its a vastly more digestible game than Chess or Go in an age where the majority of gamers have short attention spans.

introduction_of_gomoku_002
A game of Omok (more commonly referred to as Gomoku or Renju) which just came to an end. Can you spot which player won the game?

 

Rules

The basic rules of Omok are so simple that they can be summed up in just a few bullet points:

  • The winner is the first player to construct a line of five consecutive stones of their own color.
  • The black player goes first.
  • Players take turns placing one stone down at a time at any point on the board where two lines intersect.
  • Omok can be played on any size of Go board, though 15×15 is the regulation size.

The player who goes first (black) has tremendous initiative and an obvious advantage over the player who goes second. To counteract this, three restrictions are placed on the black player which do no apply to the white player. These restrictions are referred to as fouls, and occur when a player creates a double three, a double four, or an overline.

Double Three

Since an open row of three will always lead to a victory if your opponent does not place one of their stones on either end of it, a move which creates two simultaneous open lines of three should always lead to a guaranteed victory, as the opponent can’t defend against both open lines of three with a single move. As a way of equalizing black’s advantage in Omok they are restricted from making such moves.

illegal3.jpg
Moves A, B, and C are all illegal moves for black as they create two “open” lines of three. Move D is not an illegal move because it only creates only one open line of three, the other line of three is “closed”.

In the above diagram, the moves A, B, and C are examples of illegal moves for black. Move A creates open lines of three horizontally and vertically, move B does the same, and move C creates open lines of three both horizontally and diagonally. Move D, however, is not an illegal move for black. The white stone in this are makes the horizontal line “closed”, so the black player would not be creating two “open” lines of three by placing their stone there. In the case of move D, white would only be immediately forced to respond to the horizontal open line of three created by move D and is able to avoid certain defeat.

As creating two open threes in the same move is illegal for black, black’s most common route to victory is to create a 4×3 line. As white is not restricted from making a 3×3 in a single move, the 3×3 is generally white’s easiest path to victory.

illegal3disconnect.jpg

It is important to note that an open line of three can be constructed even if all three stones are not directly connected to each other. In the above diagram, moves A, B, and C are all illegal moves. Let’s examine why by looking at what happens if the black player were allowed to place a stone on move A.

illegaldisco2.jpg

If the black player places their stone on A, they can create an open line of four (guaranteeing victory on the following move) by placing their next stone at spot B, C, or D with their next move. If the white player were to defend at location B, black could create on open line of four with a move to C or D and guarantee their victory. If white were to attempt to defend at location C or D, black could create an open line of four by placing their stone at location B, likewise guaranteeing their victory. Lines of two stones, an open spot, then another stone (illustrated above by the stones on either side of spot B) demand immediate response from their opponent and are considered an “open” line of three.

In Omok, a move which creates a double three for black is referred to as a “foul” and results in an immediate loss for black. Since it is quite difficult for newer players to always identify spots which will result in a foul, I recommend spending your first few hours with Omok on an app like Dr. Gomoku. This app marks foul spots with an X and prevents the black player from placing a stone on these spots in the first place. After a few hours with the app it will become much easier to identify the location of foul spots, at which point you can try switching over from the app to a physical Go board if you’re so inclined.

Double Four

open4

Illustrated by the labeled moves in the above diagram, black is also restricted from making two open lines of four. Though not nearly as common as the double three, the double four still comes up from time to time and is an illegal move for black.

Overline

ovelrine

Illustrated by moves A and B, black is also not allowed to construct a line of stones which is six or greater in number, open or closed. In my experience this situation arises slightly more often than the double four, but not often enough to be a major hindrance to the black player.

First Move Restriction

Though I fail to see any reason for black to do otherwise, black’s first move of the game must always be placed on the middle stone of the board.

Don’t worry if this seems confusing!

If the specifics of what constitutes a foul (especially the double three) seem confusing to you, don’t worry! The basics principles of Omok are extremely simple. If you play the game in an application (such as Dr. Gomoku) it will prevent you from committing fouls in the first place and mark these spots with an X. I played my first twenty or so games of Omok without fully understanding what a foul was and was still able have a great time while doing it.

Now, on to some basic strategy.

Strategy Guide

Section 1 – Always Check for Open Threes!

Something which should be immediately apparent to any new Omok player is that an open line of three stones of the same color demands an immediate defensive response from the other player. If a player does not respond to a line of three stones by placing a stone on either side of the line (or by threatening victory with their own line of four stones in another location), the player with three open stones can place a fourth stone on either side of their line of three to create a guaranteed victory on the following turn.

In my first 100 ranked online games of Omok, I would guess that nearly half of my games were either won or lost because of a missed an open line of three stones. Though it might seem trivially easy to spot an open line of three, it cannot be overstated that missing an open line of three is a guaranteed loss if your opponent does not also miss it.

If you’re looking to quickly improve at Omok your first objective should be to never miss any open lines of three presented to you by your opponent. After several hundred games of Omok I still occasionally miss an open line in an online game, which can only be attributed to a lack of focus. At the beginning of each of your turns, always examine the 8 directions around your opponents last move to see if an open line of three was created which demands an immediate response. Ensuring you always do this will increase your win percentage at Omok by more than any other piece of strategic advice in this guide.

Taking the Initiative

Most games of Omok will reach a position where one player (most often the black player in the early and mid game) is clearly on the offensive, while the other player is clearly on defense. Creating open threes can always lead to creating a closed four (forcing your opponent to block the winning move of five with their next turn), which can be followed up by another open three, then another closed four, and so on and so forth. Being the player on the offensive allows you to hold on to the initiative of the game, a term which I’ll refer back to throughout the rest of this strategy guide.

Generally speaking, the goal of the player with initiative is to maintain it while leveraging it into a favorable position. Conversely, the goal of the player without initiative is to stop the attack of their opponent and regain initiative themselves. One way to regain the initiative as the player on defense is to maneuver into a position where your next defensive move doubles as offensive move by creating an open three.

initiative

In this overly simplified example, blocking the black open three with a move to A forces black to eventually respond to white’s open three. Though a white move to A is probably obvious to even the newest of players, I felt it nonetheless important to show an example of how a player can take initiative back from their opponent. Its as much the job of the player with initiative to avoid scenarios where the other player can take the initiative back as it is for the defensive player to find ways to regain initiative while on the defensive. In the above example, the black player should have found a way to make their open three without giving the white player the option to make their own open three with a move to A in the first place.

Though it would be rare to play an entire game of Omok in which both players maintain initiative throughout its entirety, the broader concepts of how to play when neither player has initiative are much more complicated and nuanced than how to play when one player has it. I’ll try my best to cover some basic concepts for how to play when neither player has initiative in a later section.

Planning Your Attacks

plan
All four of these moves create an open three, demanding a defensive response.

One of my biggest “level up” moments occurred when I began to grasp the concept of planning an attack. In my first few dozen games of Omok I would look at a connection of two stones such as in the picture above and assume that a move to position 1 or position 2 were my only good moves, while in reality this is far from the truth. Each of the four moves in the picture above could be correct move depending on where on the board you are planning to build towards your next attack.

plan2
The sequence of numbered moves forces white to use their turn to put a stone in a potentially useless position.

In a vacuum (and especially in the early game) it’s generally favorable to connect your stones together with a move like 1 or 2, as these moves lead to an easy follow-up move of creating a closed four (forcing another response from your opponent while keeping initiative). Creating a closed four (move 3) is a forcing move for the other player and is often a good choice, as a closed four can force your opponent to place a stone in an area which is disconnected from their other stones while simultaneously connecting a stone of your own next to its friends.

That said, blindly connecting forcing moves together by creating chains of open threes and closed fours is generally not a winning strategy. You might be able to luck your way into a victory against a novice opponent, but a smart opponent should be able to leverage your lack of foresight into an easy trap.

plan3

In the above example, a newer player might look at the board and see that moves C and B create a forcing move for black, then use their turn to create the closed four. A more advanced player would look at this board and see that if black could first get a stone at position A, a move to B would force white to defend the closed four at C while leaving black with an open 3 (a guaranteed win). To put it another way, if black can find a way to get a stone on position A before giving white the opportunity to defend against their attack, black can put a stone on B and guarantee their victory. In this example position A is a what I refer to as a “critical point”.

In the mid and late game, identifying the location of critical points and creating an opportunity to place a stone there is what will separate the beginning players from the intermediate and advanced ones. To create this opportunity one must maintain initiative throughout the entire process, as a single turn without initiative could result in your opponent laying an unwelcome stone down on your critical point. It’s a good rule of thumb that the best possible location to place a stone for the offensive player is likely the best possible move for the defensive player as well.

So how do you go about identifying critical points? The answer to this question is easier than you may think. Look for double three fouls! As black, any double three foul can be transformed into a winning open three + four combination by finding a way to put one extra stone along a potential point of attack. As white, finding critical points is even easier as you often need one fewer stone than black to create a critical point, since you can simply create a double three instead of an open three + open four combination.

Though not all critical points stem from double three fouls, all double three fouls can lead to a critical point.

criticalpoint1
A is a critical point in this board, as it creates a winning open four/open three combo for black at spot B.

Though you can’t perfectly control which spot your opponents will place their defensive stones, its generally safe to assume that your opponents will choose to place their defensive stones in the spot you least want them to. This implies that you should never expose your critical points as a potential defensive location of an open three or a closed four.

Controlling Space on Defense

Though it may seem difficult to construct an advantage as the defensive player, if you can properly control the flow of the pieces on the board it is possible to gain tremendous advantages as the defensive player.

deadstones
The dead stones for black are marked with a letter, and the dead stones for white are marked with a number.

In the above game white was able to successfully defend against black’s initial attacks. At the end of the attack, black has a number of stones (marked with letters) which are completely useless for the rest of the game, which I call “dead stones”. Dead stones have no possibility of connecting with future “live” stones, and are effectively empty space on the board. In addition to the dead stones labeled above, black also has a number of stones which are very nearly dead, such as the black stone between A and C which could killed with a white move to its northwest spot. White, on the other hand, is only left with one dead stone at the end of this exchange and has a number of exciting attacking opportunities it can leverage in the near future. White is heavily favored to win from this position.

There are many situations in Omok where the best decision for the defender is to try and leave their opponent with more dead stones than they leave themselves instead of racing to try and seize initiative back from the offensive player. Placing defensive stones in a way that allows the defender to box in an attack is often a stronger move than attempting to connect defensive stones together in an attempt to quickly retake initiative. Initiative will always be taken back by the defensive player if the offender’s attack is successfully thwarted, and the defender will find his or herself in a more advantageous position than their opponent if they were able to kill their opponents stones along the way to that goal.

Conversely, if an attacker feels that their attack is going nowhere in a hurry they should know when to shift their focus away from their fruitless attack and start working towards keeping the stones they have already committed to the board alive. Initiative is a powerful tool, but true control of the game can quickly slip away from the player with initiative if their attack is doomed to fail.

defense
Move B connects two of white’s stones together for a potential attacking move in the future. Move A cuts the black player off from many offensive opportunities in the lower portion of the board, but strands the white stone in the upper left corner. Its not always clear which move is stronger.

Its not always obvious when a defender should be aiming to kill their opponents stones and when they should be aiming to connect their defensive stones in an attempt to regain initiative. Knowing when to aim for one or the other is as more of an art than a science. Speaking for myself, I’m a much stronger defensive player than offensive one and have a higher win percentage as white than black. Knowing my personal preference for defense over offense, I generally choose to control space and aim to kill my opponents stones when the decision to defend or take back initiative isn’t immediately obvious. You’ll get a feel for which kinds of stone patterns demand more defensive respect and which situations warrant move offensively-minded defense as you get more and more games under your belt. Your experience, combined with your own personal preferences, will be your guide in these kinds of unclear situations.

What should you look for when nobody has initiative?

I find Omok to be at its most challenging when neither player has initiative. When nobody has initiative, broader “art of war” type concepts start to take shape on the board. Attempting to exhaustively cover these concepts in a basic strategy article would be fool’s errand (at least partially due to the fact that I can’t claim to be anywhere close to understanding all of these concepts myself), but there are many things you can look out for to help you in these situations.

Moves which both attack and defend

noinitiative1

In the above example it’s black’s turn to place a stone. They have the ability to make an open three on the left side of the board but that attack doesn’t promise to amount to much. At this point initiative has been lost for black. What’s their best move?

noinitiative2

The move marked with a triangle accomplishes two things at the same time. It both blocks a potential open three from white and connects with another black stone (the one its northwest) to create a potential open three on a future turn. Try to keep an eye out for moves like this which advance your position both offensively and defensively.

Create potential connections

noinitiative3

Black and white have battled back and forth, each briefly holding initiative. Itss now white’s turn to play. They have the option to create an open three at positions A and B, or a closed four at positions C or D. But are these moves really the best white can do in this position?

noinitiative4

Another option for white is to threaten to create two potential open threes on the following turn. This accomplishes several things. Firstly, it places a stone in what looks to be a more critical location of the board than A, B, C, or D cloud, which blocks potential future attacks from black as the action on the board moves to the north. As this move also potentially allows white to connect two different open threes on the following turn, this move plays a bit of offense and defense at the same time (just like I alluded to in the previous section).

When neither player has initiative look to find moves which serve to connect stones played in previous turns which would otherwise be far away from the action. Moves which threaten to connect multiple stones from previous turns often prove to be far more useful than moves which connect only one stone from a more recent turn.

Take away your opponents best move

noinitiative5

In this scenario black attempted an attack and white was able to easily deflect it. There are no locations on the board which threaten an open three for black, so initiative is lost. White is threatening several open threes on the following turn, so its probably a smart move for black to play some preemptive defense. How do you find the best location on the board to defend in this scenario? Easy! Look for your opponents best move and take it away from them.

noinitiative6

The move to the marked location would have been a very strong move for white on the following turn. It would have created an open three diagonally while simultaneously creating two open twos, both of which threaten to expand into dangerous positions for black as white seizes the initiative. Though there is no way in this scenario for black to stop white from creating an open three on the following turn, the move to the marked location could mitigate a potential disaster and allow the black player a chance to get back in the game.

Cheeky Tactics

There are a few prebaked strategies I have come across which I felt were worth mentioning before I closed up this guide. If nothing is else is working for you, try implementing these cheeky tactics and see if they can lead you to victory.

The Bee’s House

The name for this strategy was coined by my girlfriend, who is still learning English and often comes up with funny names for words she doesn’t know. The “bee’s house” is a great way to steal a few games against other beginners, and is difficult to defend against if you’ve never seen it before. This tactic looks to place as many stones as possible along diagonal lines, threatening to eventually close the spaces between the connecting diagonals to create a winning scenario.

beeshouse

The above board is a demonstration of a bee’s house tactic. You can see that the black player has attempted to place as many stones as possible along diagonal lines in the southwest of the board, and continues to make open threes and closed fours along diagonals for as long as possible. Though these diagonals will never lead to a connection of 5 on their own, the middle points created between the diagonals can easily lead to a victory of your opponent gave you too much room to operate. Just be careful not to commit a double three foul!

beeshouse2

In the 5 move sequence demonstrated above, black was easily able to construct a chain of closed fours which led to an open four (and victory) from the previous board state.

To execute the bee’s house, simply look to place moves only along diagonals and build as large an area as possible in this manner. You’ll eventually run out of room, but if your opponent isn’t quick to sniff out your plan (like in the above demonstration) you should be able to string together enough diagonal open threes and closed fours to create a winning sequence. As the white player, the bee’s house is even more effective as it easily leads to double threes.

The Net

This tactic comes courtesy of a more advanced strategy guide than mine, titled Renju for Beginners, I found the guide to be very useful, though a tad confusing at times and not perfectly suited for true beginners of Omok/Renju/Gomoku.

thenet.jpg

The net is a strategy for the white player which allows them to kill a large number of black stones while at the same time using the minimal number of stones possible to accomplish it. By placing stones along an L-shaped grid as demonstrated above, black is unable to create any chains of five inside the net. The net, of course, can be “broken” if the black player places a stone at any position along the net, but until such a move occurs the white player stands to kill a large number of black stones with a minimal amount of effort. Experienced players will sniff out this strategy pretty quickly, but perhaps not before the damage has already been done. Speaking from personal experience, the player who first used the net against me in an online match was able to easy trap me in it.

Conclusion

The world of Omok might not be as big or as deep as the worlds of Chess an Go, but I believe the game has the potential to appeal to a much wider audience in this day and age. Its easy to learn, the games are much faster paced and easier to digest than Chess or Go, and apps like Dr. Gomoku make it extremely accessible to pick up the game and play in an competitive environment.

See you online!

– Aleco Pors

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s