How to play Ojeom

What the game is

Players take turns placing stones, and the first to connect five in a row, column or diagonal wins. On top of that comes capturing. When exactly two opponent stones sit between two of yours, those two leave the board. Connecting five and capturing by sandwiching are both old common formats, and a board game that combined the two was already on sale in the 1970s.

Exactly how a capture works

Each of the eight directions is checked separately. From the square you played, if there are exactly two opponent stones in a row and then one of your own, those two leave the board. A single move can capture in several directions at once. The count has to be exactly two: three stones in between do nothing at all.

So you do not stand two in a row

If you place two stones side by side to grow a line, both ends of that pair become a weakness. When the opponent already holds one end, a single stone at the other end takes your pair away. So it is safer to leave a gap or stand three together, because three does not meet the capture condition.

An open four is still frightening

Four in a line with both ends empty still wins: block one end and five comes on the other. One thing is added here though. If the middle of that four is captured, the line breaks apart. So when you build a four, check whether that line can be captured. Our opponent bot cuts the square that would create an open four before anything else.

A capture is a gain by itself

When opponent stones leave, their line gets shorter too, so one move attacks and defends at once. In our simulation the side that counted captures beat the side that only grew its own line. On a day with the ten stones rule, the capture count itself becomes a way to win, and the math changes again.

Where people get stuck

The first stone can only go in the center, and black's second stone has to be outside the five squares around the center. It is a convention that takes some of the first player's edge away. And there is no undo. The same day is always the same board, so replaying from the start shows exactly where it turned.

How to choose the day's rule

Three of the 6 candidate rules are drawn for the day. You ban one and pick one. Four of the six change capturing directly.

• No diagonal capture You cannot capture along a diagonal. Only rows and columns count.
• Three also captured Three opponent stones between two of yours also leave the board.
• Turn them over Captured stones stay on the board and become yours.
• Ten stones also win Capturing ten opponent stones wins even without five in a row.
• No outer ring The outermost line of the board is off limits.
• Only three captured Two opponent stones between two of yours are safe; it takes three to capture.

Think about the ban first. Removing the one of the three that hurts you most is worth more than the pick. Three also captured makes captures fire far more often and the board turns rough, while Only three captured does the opposite and makes standing two safe. If you like standing stones side by side, ban the first; if you like leaving gaps, ban the second.

Same day, same board

There is no randomness. The candidates are set by the date, and the opponent always plays the same moves on the same board. So if you replay a lost game you can see exactly where it turned. A loss is not bad luck but the wrong order of moves, and memorizing it makes you better.

No rule ever leaves you without a legal square. A narrowing rule that blocks the whole board would be an accident, not a rule, so if narrowing would leave nothing, it is not applied.

About the name

Ojeom is a name we made up; it means connecting five points. Connecting five and capturing by sandwiching are both common formats that anyone may build on, but the name of the 1970s commercial board game that used the same pair of mechanics may be a trademark, so it appears nowhere on these pages. The rule descriptions were written by us from our own code rather than carried over from that product.