Every gomoku player learns to block within the first few games. You see three or four of the opponent's stones in a row, you put one of yours at the end, and the danger seems to pass. Then, a few moves later, a different line suddenly has four and you have no answer. The block did not fail because you were slow. It failed because you blocked the wrong end, or because the block did nothing except block.
This article is about blocking well: choosing the end that matters, choosing a block that also helps you, and knowing when a line is not yet worth a stone. It applies to both rule sets on this site, the Vietnamese blocked ends rule and freestyle, with a few differences noted along the way.
What a block actually does
A line becomes dangerous when it can grow into five. A block removes some or all of the room it needs. That gives two simple questions to ask about any enemy line:
- How many open ends does it have? An open end is an empty point right next to the line, with room beyond it.
- How many moves does it need to become five?
An open three (three in a row, both ends open, with room beyond) needs one move to become an open four, and an open four cannot be stopped because it threatens to become five at both ends. That is why an open three must usually be answered at once. A closed three, with one end already blocked, needs two moves to become a threat you cannot answer, so it can often wait.
Which end to block
Beginners usually block the end nearest to them, or the end they happened to be looking at. Better players ask which end is more useful for the attacker and close that one.
Look past the line itself. On one side there may be other X stones that the line could connect to later, or a diagonal that crosses the same point. On the other side there may be an edge of the board or an O stone that already limits growth. Block the side where the attacker has more friends.
A second question is which end is more useful for you. A blocking stone is still your stone. If one end sits next to your own two stones on a diagonal, blocking there turns your defence into a small threat of your own. Your opponent must now think about your line as well as theirs, and that is often the moment the initiative changes hands.
When both ends look equal, prefer the one closer to the centre. Stones near the centre touch more lines and are worth more later in the game.
Blocking under the blocked ends rule
In Vietnamese caro with the blocked ends rule, a line of five does not win if both of its ends are already closed by the opponent. This changes blocking in two ways.
First, it makes blocking early more valuable. A line that you have closed at one end can still win, but only by extending toward the open end. If you later close that end too, even five stones will not count.
Second, it creates a trap for the attacker. Players used to freestyle sometimes build four in a row between two enemy stones and expect the fifth stone to win. Under this rule it does not. When you defend, keep track of which enemy lines are already closed at one end; those lines are half-dead, and a single well-placed stone can kill them completely.
On the freestyle setting, any five in a row wins, so a closed four is still a real threat: you must block its one open end immediately.
When not to block yet
Not every line needs a stone right now. Spending a move on a line that is not dangerous gives your opponent a free move elsewhere, and free moves are how double threats are built.
A useful rule of thumb:
- Open three: block now, almost always.
- Four of any kind: block now, always. There is no choice.
- Closed three: usually leave it, unless blocking it also builds your own line.
- Open two: leave it, unless the two is aimed at a point where another enemy line crosses.
The exception to all of this is the point where two enemy lines cross. If the opponent could play one stone there and create two threats at once, that point is more important than any single line. Occupy it before it becomes a four-three or a three-three.
Blocks that also attack
The best defensive move in gomoku often looks like an attacking move. Suppose you have two stones on a diagonal, and the enemy open three has one end sitting exactly where your diagonal would become three. Blocking there costs you nothing: the enemy line is closed, and you now have a three of your own that the opponent must watch.
Train yourself to look for these points before choosing a block. Before you place a defensive stone, check the eight directions from each candidate point and count your own stones on those lines. The candidate that touches the most of your stones is usually the right one.
A three-second habit before every block
Most lost games are lost to a threat you did not see, not to one you saw and blocked badly. Before each move, spend a few seconds on this checklist:
- Did the opponent's last stone create a four anywhere? If yes, block it; nothing else matters.
- Did it create an open three? If yes, block one end, choosing the more useful one.
- Did it create two threes, or a four and a three? If yes, you are probably already lost, so look for your own four to force play instead.
- If none of the above, is there a crossing point where one enemy stone would create two threats? Consider occupying it.
It feels slow at first. After a few dozen games it becomes automatic, and you will notice the difference in how rarely you are surprised.
Practise against the computer
The board on the home page lets you play the computer at four levels and switch between blocked ends and freestyle rules. Blocking practice works best at the second or third level: strong enough to punish a missed three, gentle enough that you see the pattern before the game ends. When you lose, use undo to step back to the move before the winning threat appeared and find the block you missed. The defense section has more articles on specific situations.
Frequently asked questions
Should I always block an open three immediately?
Almost always. The exception is when you have a four of your own to play first; forcing moves come before blocks.
Which end of the line should I block?
The end that is more useful to the attacker, and if possible the end where your blocking stone also joins your own stones.
Does blocking work differently under the blocked ends rule?
Yes. A five that is closed at both ends does not win, so lines you have already closed at one end become much weaker. Blocking early pays off more.
Why do I keep losing even though I block every three?
Usually because the opponent is building at crossing points where one stone creates two threats. Watch those points, not just the lines.
Is it bad to block too early?
It can be. Spending a stone on a closed three or a scattered two gives the opponent a free move to build elsewhere.
