Hidden Rules (隠された掟ゲーム) is a game played by the Ougatou twins in Volume 6 (Twin) in their gambling den. Mary Saotome and Mikura Sado play this game against the sisters and Ouri Shimotsukiuri.
Rules[]
The game is played with a deck of 52 cards for each player and a deck of 50 "rule cards" (掟カード). Rule cards determine which cards are allowed to be played during the round. For example, a rule card may establish that players have to alternate between hearts and spades, or that only cards with odd numbers are allowed. Without being shown said card, the other players have to guess the hidden rule during the game.
First, a player (the dealer) draws three rule cards and chooses one. Afterward, they play a card and declare if it matches the hidden rule. One after the other, the players have to play a card each, trying to conform to the rule; the dealer will personally declare if a card is allowed or not. If a card matches, the player who used it gains one point. When a player has realized the pattern of the hidden rule, they can try stating it: if they are correct, they gain ten points. However, every player can only try to guess the rule once per round. If nobody is able to guess the hidden rule after ten turns or two trips around the table, the dealer is penalized and loses every point they gained during the match plus another ten points.
Game[]
After the sisters explain the rules, round one begins. Houko starts by picking a rule card, to which she chooses spade (multiple of 3) --> club (multiple of 3) --> heart (multiple of 3) --> diamond (multiple of 3). She plays the first card, 3 of spades, which is legal. Mikura plays a 2 of hearts, which is illegal. Naoko plays a 6 of clubs, which is legal. Ouri plays a 3 of hearts, which is illegal. Mary plays a 8 of hearts, which is illegal. Houko plays a queen of hearts, which is legal. Naoko plays a 5 of hearts, which is illegal. Ouri plays a 3 of diamonds, which is illegal. Mary plays a 9 of diamonds, which is illegal. Naoko attempts a reveal, and gets it correct. Houko has 11 points, Naoko has 10 points, and Mary, Ouri, and Mikura all have 0 points.
Round two begins. Mikura picks a rule card, and she chooses hearts only. She plays a 5 of diamonds, which is illegal. Naoko plays a 6 of hearts, which is legal. Ouri plays an 8 of hearts, which is legal. Naoko attempts a reveal, but gets it wrong. Mary then plays a 7 of hearts, which is legal. Mary then attempts a reveal, and gets it correct.
Round three begins. Naoko picks a rule card, and she chooses heart (odd) --> spade (odd) --> club (multiple of 3). She plays a 6 of clubs, which is illegal. Ouri plays a kind of diamonds, which is illegal. Mary plays a 2 of hearts, which is illegal. The game continues, until Houko attempts a reveal, and gets it correct. Houko and Naoko have 22 points, Mary has 11 points, Mikura has 5 points, and Ouri has 2 points.
Round four begins. Ouri picks a rule card, and she chooses diamonds --> clubs (even). She plays a 10 of diamonds, which is legal. Mary plays an 8 of diamonds, which is illegal. Houko plays a 9 of hearts, which is illegal. Mikura plays a 4 of clubs, which is legal. Naoko plays a 7 of diamonds, which is legal. Ouri plays a jack of spades, which is illegal. Houko and Mary simultaneously call for a reveal, and Houko lets Mary attempt a reveal. Mary gets it correct.
Round five begins. Mary picks a rule card, and she chooses heart --> spade (even). She plays a 2 of hearts, which is legal. Houko plays a jack of hearts, which is illegal. Mikura plays a 6 of spades, which is legal. Naoko plays a 9 of hearts, which is legal. Ouri plays a 5 of diamonds, which is illegal. Mary plays a 4 of clubs, which is illegal. Houko plays a 5 of spades, which is illegal. Houko attempts a reveal, but gets it wrong. Mikura attempts a reveal, and gets it correct.
Round six begins. Houko picks a rule card, and she chooses club (multiple of 3) --> heart --> club (even) --> diamond (odd). She plays an illegal card. Mikura plays an ace of heart, which is illegal. Naoko plays a 3 of clubs, which is legal. Ouri plays a 6 of hearts, which is legal. Ouri then attempts a reveal, and gets it correct.
Round seven begins. Mikura picks a rule card, and she chooses diamond (odd) --> heart (odd). After a few turns, Mary plays a 9 of diamonds, which is legal. Houko plays a 8 of hearts, which is illegal. Mikura plays a 7 of clubs, which is illegal. Ouri then attempts a reveal, and gets it correct.
Round eight begins. Naoko picks a rule card, and she chooses club (odd) --> spade (odd) --> diamond (odd) --> club (odd). Ouri attempts a reveal, and gets it correct. Ouri has 40 points, Mary has 30 points, Naoko and Houko have 26 points, and Mikura has 24 points.
Round nine begins. Ouri picks a rule card, and she chooses clubs only. She plays a 2 of clubs, which is legal. Mary plays a queen of clubs, which is legal. Naoko, even though it's not her turn, plays a 4 of clubs, which is legal. Houko plays an 8 of clubs, which is legal. Mikura plays a 10 of clubs, which is legal. The round continues and finishes with no reveals, causing Ouri to lose 10 points. The round end with Mary having 32 points, Ouri having 30 points, Naoko and Houko having 28 points, and Mikura having 26 points.
Round ten, the final round, begins. Mary picks a rule card, and she chooses a hearts only card. Mary gives out the hint that the card is based on a single suit and there are no restrictions. She also tells Mikura that the suit is the suit her royal flush in the last round of Doubt Poker. Mikura then attempts a reveal and gets it correct.
The game ends with Mikura in first with 36 points, Mary in second with 32 points, Ouri in third with 30 points, and Houko and Naoko both in forth with 28 points. Mary and Mikura then collect their 5 million yen for winning, and Mikura gives it all to Mary so she can pay off her debt to Sachiko Juraku.