[Glossary of Mahjong Terms #6] Mahjong Sets (Mentsu), Menzen/Open Concepts/Riichi/Tenpai Basics

By TAKEYuichi

October 13, 2025 3 min read

Mahjong Sets (Mentsu), Menzen/Open Concepts & Riichi/Tenpai Basics

Hand structure and readiness are the backbone of Riichi mahjong. This guide explains tile sets (mentsu), closed vs. open-hand concepts, and riichi/tenpai fundamentals. Learn these terms to read shapes quickly and make better decisions at the table.

1. Sets & Blocks (面子・ブロック)

“Mentsu” are the building blocks of a winning hand. Understanding each set and near-complete block helps you evaluate shape quality and improvement paths.

面子 / Mentsu: A completed set: either a sequence (順子) or a triplet/quad (刻子/槓子). A standard winning hand typically consists of four mentsu plus one pair. structure
順子 / Shuntsu (Sequence): Three consecutive suit tiles (e.g., 4–5–6 in one suit). Honors cannot form sequences. set
刻子 / Kōtsu (Triplet): Three identical tiles (e.g., 7–7–7). set
槓子 / Kantsu (Quad): Four identical tiles; exposes extra dora indicators depending on the type of kan. set
雀頭 / Jantō (Head): The pair that completes a standard hand structure. pair
対子 / Toitsu (Pair): Two identical tiles. May serve as the head or become a triplet/quad. pair
塔子 / Taatsu (Incomplete Block): A 2-tile block that is one tile away from a sequence: e.g., ryanmen (open-ended 4–5), kanchan (gap 4–6), penchan (edge 1–2 or 8–9). block
暗刻 / Ankō (Concealed Triplet): A triplet formed without claiming a discard. concealed
明刻 / Minkō (Open Triplet): A triplet formed by calling a discard (pon). open
暗槓 / Ankan (Closed Kan): A quad formed entirely from tiles in your hand (drawing the fourth yourself). concealed
明槓 / Minkan (Open Kan): A quad completed by calling a discard to your triplet. open
加槓 / Kakan (Added Kan): Upgrading an exposed triplet to a quad by adding the fourth tile you draw. upgrade

2. Menzen & Open-Hand Concepts (門前・食い系)

Choosing to stay closed (menzen) or to open your hand affects speed, value, and available yaku. Know how rule variations change yaku values.

門前 / Menzen (Closed Hand): Not calling discards; all tiles are self-drawn. Some yaku require a fully closed hand. closed
門前役 / Menzen-yaku: Yaku that require the hand to remain closed (e.g., riichi-only hands). closed
副露 / Fūro (Calling / Open Meld): Exposing a set by claiming an opponent’s discard (pon/chi/minkan). Speeds up completion but often lowers value. open
喰い下がり / Kuisagari: Rule where certain yaku lose 1 han in value when the hand is open. rule
喰いタン / Kuitan (Open Tanyao): Completing 2–8 sequences with open melds (if allowed by rules). open
喰い替え / Kuikae: Immediately discarding a tile you just claimed to form a sequence, or swapping with a related tile; treatment varies by house rules and is often restricted. rule

3. Riichi & Tenpai Concepts (立直・テンパイ)

Readiness and declaration mechanics affect both pressure and value. Master these to time your pushes and folds.

立直 / Riichi: Declaring ready with a closed hand by placing a 1000-point stick; you must win without changing your hand structure. declare
ダブル立直 / Double Riichi: Declaring riichi on your first turn before any calls are made; higher value than normal riichi. declare
一発 / Ippatsu: Winning within one uninterrupted turn cycle after declaring riichi (no calls interrupting). bonus
テンパイ / Tenpai (Ready Hand): Being one tile away from a legal win. state
ノーテン / Noten: Not being in tenpai at the end of the hand; may require paying penalty points depending on rules. state
フリテン / Furiten: A state where you cannot win by ron because a winning tile is in your own discards or all remaining waits are in your discard row; tsumo is still allowed. restriction
形式テンパイ / Keishiki Tenpai (Formal Tenpai): Technically tenpai at hand end (for noten payments), even if the hand cannot actually win due to restrictions like furiten. state
向聴 / Shanten: The number of steps away from tenpai (e.g., 1-shanten, 2-shanten). Lower is closer to ready. metric
受け入れ / Ukeire: The total number of tiles that improve your hand (advance shanten or complete the win). count
待ち / Machi (Wait): The tile(s) you need to win when in tenpai. (Detailed wait types are covered in the “Waits” article.) win

Tip: Learn to evaluate taatsu quality and ukeire quickly. It speeds up your decision-making on whether to stay menzen for value or open for speed—and when to declare riichi.