The game of Zendo

Circa 2013, Kory Heath wrote a great long-form memoir describing the design history of Zendo.

Zendo is one of those creations that makes people slap their foreheads and say “why didn’t I think of that?” It’s just so darned simple. Surely the idea must have sprung fully-formed into its designer’s brain, just ready to be played? What else could possibly have been required? […]

Of course, that’s not really how the creative process works. […] The first versions of Zendo were clunky, ugly, and complex. The playtesting team and I spent literally hundreds of hours, over a period of many months, molding the game into its final form. This essay tells the story of that process—sometimes in excruciating detail.

If you haven’t seen Zendo before, the official rules are here; here’s a quick summary. The quickest summary is:

Interactive Bongard problems with Icehouse pieces.

The game requires several sets of Icehouse pieces, a.k.a. Looney Pyramids (yes, the same pieces with which you play Homeworlds! but you ought to have a lot more of them). From this global pool, the players will be building agglomerations or arrangements which we call “koans.” One player, designated the Master, will have secretly devised a rule that, applied to any koan, definitively classifies it as “having the Buddha-nature” or “not having the Buddha-nature.” The other players, the Students, are trying to guess the Master’s rule. The game requires a supply of white stones (to mark koans that have the Buddha-nature), a supply of black stones (to mark those that don’t), and a supply of “guessing stones” preferably of a third color.

Those are the traditional accoutrements; but any kind of components will do, as long as you have enough. I’ve played Zendo at a wedding reception using plastic utensils and napkins. You could even do it with nothing more than paper and pencil, in which case you could also do away with the black and white stones and describe it as “Bongard problems without Icehouse pieces.”

One player is designated the Master. The Master thinks of a secret rule for classifying koans that have the Buddha-nature; for example, “A koan has the Buddha-nature if it contains at least two red pieces,” or “A koan has the Buddha-nature if all of its pieces are different sizes,” or “A koan has the Buddha-nature if no piece points at any other piece.” (The rule must be about the koan; it would be cheating to make a rule like “A koan has the Buddha-nature if it’s on the north side of the table” or “Every second koan created has the Buddha-nature.”)

The Master then publicly creates two koans — one with the Buddha-nature (according to his rule), and one without the Buddha-nature — and labels them with a white and a black stone, respectively.

For example: Alice is designated as Master. Alice secretly thinks of the rule “A koan has the Buddha-nature if it contains an odd number of green pieces.” Alice makes a koan consisting of three small green pyramids lying flat in a triangle with their points together, and marks it with a white stone. Alice makes a koan consisting of two upright medium red pyramids, and marks it with a black stone. Alice looks expectantly at the Student to her left.

The rest of the game consists of the Students taking turns in order. On your turn, you must make a koan. (If you need pieces that aren’t available, you may ask the Master to get them for you by breaking down previous koans.) Having made your koan, you have two options:

Say “Master?” — The Master marks your koan with a white stone if it has the Buddha-nature, or with a black stone otherwise.

For example: Bob makes a koan consisting of three small red pyramids lying flat with their points together, and says “Master?” Alice marks Bob’s koan with a black stone.

For example: Carol stacks a small red on top of a medium yellow on top of a large green. “Master?” Alice marks Carol’s koan with a white stone.

Say “Mondo!” — Before the Master marks your koan, each Student conceals in their hand a white or black stone, indicating their guess as to whether your koan will be judged to have the Buddha-nature. Each Student who guesses correctly receives a guessing stone.

For example: Dave places a single medium green pyramid upright on the table and says “Mondo!” Bob, Carol, and Dave reveal their guesses: Bob guess black, Carol and Dave guess white. Alice marks Dave’s koan with a white stone and awards guessing stones to Carol and Dave (but not Bob).

At the end of your own turn, you may (but do not have to) spend guessing stones to make guesses at the Master’s rule. Each guess costs one guessing stone. The Master may ask clarifying questions if necessary. If the Master judges that you have in fact guessed the secret rule, then you win! If your guess wasn’t quite right, then the Master must show you a koan that disproves your rule. (The Master should fully understand your rule; that’s what the clarifying questions are for. By the way, the players are also allowed to ask clarifying questions of the Master about the properties of individual koans, during the game.)

For example: Dave returns his guessing stone and says, “Master, I think a koan has the Buddha-nature if it contains any green pieces.”

Alice shakes her head and constructs a stack of two medium greens on top of two medium yellows. She marks it with a black stone.

The disproving koan might be one that unexpectedly fails to have the Buddha-nature (as in the above example), or it might be one that unexpectedly has the Buddha-nature when the Student thought it wouldn’t.

Bob makes two adjacent stacks with small greens on top of small yellows. “Mondo!”

Dave asks, “Are those two stacks touching?” Alice peers at them closely and answers, “Yes, they’re touching.” Bob pointedly nudges the stacks apart.

Everyone guesses black; everyone receives another guessing stone.

Bob guesses, “Master, I think a koan has the Buddha-nature if it contains a green piece touching the table.” Alice asks, “Do you mean exactly one green piece touching the table, or any number of green pieces touching the table?” Dave helpfully gestures to Alice’s first koan. “Any number,” says Bob.

Alice shakes her head and stacks a small green on top of a medium yellow on top of a large red — the inverse of Carol’s koan. She marks it with a white stone.

Bob now spends his second guessing stone. “Master, I think a koan has the Buddha-nature if the number of green pieces in it is an odd number.” Alice nods in approval, and Bob becomes Master for the next round!

Kory Heath’s page has a great collection of tips for playing the Master and an even greater collection of tips for playing the Student.