1. One Board, Three Different Goals
In Go, players take turns placing stones on empty crossing points. The board is a 19-by-19 grid, and you can think of it as land to be shared between the two players. Once a stone is placed, it stays put, unless it is surrounded and captured. The aim is to surround more territory than your opponent [2].
Gomoku uses the same stones and the same crossing points, but the aim is much simpler. Players take turns, and the first to make an unbroken line of five stones of their own color, across, up and down, or on a diagonal, wins [5]. Nothing is captured and nothing is counted at the end. Because stones are not moved or removed, gomoku can even be played on paper with a pencil [5].
Renju is a version of gomoku for serious competition. It was named renju by the Japanese journalist Ruikou Kuroiwa on December 6, 1899, in a newspaper called Yorozu Choho. Wikipedia says the name means "connected pearls" in Japanese [4]. So a single set of stones supports a game of surrounding territory and a game of line-making. What makes renju different from plain gomoku?
2. How Renju Changes Gomoku: A Smaller Board and Rules for Black
First, the board. The official international renju rules call for a board with 15 lines in each direction, which makes 225 crossing points [1]. A Go board has 19 lines each way and 361 crossing points. So a renju board has four fewer lines in each direction than a Go board, and uses about 62% as many points as Go (225 divided by 361). Gomoku is also commonly played on a 15-by-15 board today, although a 19-by-19 board used to be standard [5].
Second, the rules. Wikipedia's gomoku page describes a first-move advantage in plain gomoku when nothing restricts the first player [5]. Renju was built to weaken that advantage by adding special restrictions for Black, who plays first [4]. The player using Black stones may not make these moves:
- Double three: one stone that makes two open lines of three at once.
- Double four: one stone that makes two lines of four at once.
- Overline: six or more stones in an unbroken row [4].
If Black plays a forbidden move, White wins. There is one exception: if the forbidden move also completes five in a row, Black still wins [4]. The restrictions apply only to Black, not to White.
How big is the first-move advantage? In the 1989 and 1991 gomoku world championships, which already limited the first player's opening moves, Black still won about 67% of the games, according to Wikipedia. The same page lists Swap2, an opening in which the second player gets a say in which color to play, as one of the tournament opening rules meant to balance the start [5]. Renju's restrictions and Swap2 aim at the same problem: moving first helps a lot.
3. Where Did Five-in-a-Row Come From, and Who Gave It the Name Renju?
Nobody knows exactly where five in a row began. Wikipedia traces it to Japan's Edo period, the mid-1700s, when a merchant said to be very good at the game helped it spread among ordinary people [5]. The same page names an 1856 book as the earliest published one on gomoku that can be verified, a collection of opening patterns titled Gomoku Jōseki Collection [5]. The name "gomoku" comes from Japanese: go means five, moku is a counting word for pieces, and narabe means lining up [5].
Renju got its name in 1899, when Kuroiwa proposed it in a newspaper [4]. The current international rules, with the 15-line board, were decided much later by the Renju International Federation, on May 2, 1996, with a small correction in 1998 [1].
4. What Happens to Go Itself on a Smaller Board?
Smaller boards are not only for other games. Go players use them too. The British Go Association says the normal size is 19 by 19 but that smaller boards are possible. It says a quick game on a 13-by-13 board keeps the essential character of Go, and it recommends that beginners learn on a 9-by-9 board, then move up to 13 by 13. It suggests trying 19 by 19 only once you can finish a whole game on a smaller board within 15 minutes and feel comfortable with the main ideas [3].
The numbers show why. A 9-by-9 board has 81 crossing points, a 13-by-13 board has 169, and a 19-by-19 board has 361. A 9-by-9 board is only about 22% as big as the standard board (81 divided by 361), and a 13-by-13 board is about 47% (169 divided by 361). Fewer places to play means fewer choices on every turn, so a beginner can finish a game and see how it ended.
5. Was the Go Board Always 19 Lines?
No. The British Go Association says Go probably began in China some 3,000 to 4,000 years ago, though its true origins are unknown [6]. The oldest boards that survive come from digs, and they show that early boards were smaller than the modern 19-by-19 grid. Wikipedia, summarizing a book by Marco Milone, lists a 17-by-17 board from the Han dynasty found in 1954, a 13-by-13 board from the Liao dynasty found in 1977, and a 19-by-19 board from the Sui dynasty found in 1954 [7]. Because I only found this list in Wikipedia and not in the original book, treat these exact finds as tentative, since I could not check the original book.
Either way, the same lesson holds: the number of lines is a choice, and people have made different choices at different times. Renju is one more example of that choice, made for a different game.
6. How Can You Test Board Size Yourself?
You do not need a Go set. On grid paper, treat the places where lines cross as the points, and take turns drawing O and X. The first person to get five in a row, across, up and down, or on a diagonal, wins.
Play one game in a 9-by-9 area and another in a 15-by-15 area. Then ask yourself some questions. Which game ended sooner? In which one did you have to think about more places at once? Did going first help you more on one of the boards? If you have a Go board and stones at home, try the same experiment with an adult. Go stones are small, so keep them away from younger children who might put them in their mouths.
To read more, the Renju International Federation publishes its rules online, and the British Go Association has a short guide to playing Go on small boards (see the sources below).