Naki strategy
The option to call tiles is a discretionary play which warrants a least a little observation and evaluation. With any given hand, a player must decide to call tiles or not. This page largely considers pon and chi, not kan.
Assessing calls
Like any decision in the game, opening your hand has advantages and disadvantages.
Advantages
- Allows your hand to complete faster. Sequences can be completed with twice the number of tiles (your draws, and the left opponent's discards). Triplets can be completed with four times the number of tiles (your draws, anyone else's discards).
- Can allow you to complete a hand you otherwise wouldn't have completed at all. If you have 12-man, and the last 3-man is discarded, you must either call or not complete the sequence.
- Can allow you to confirm a yaku. Yakuhai, for instance, are hard to complete without calling pon.
- It is hard for opponents to know whether an open hand is complete or not.
- Disrupts ippatsu, as well as tenhou, chiihou, double riichi, the option to call kyuushu kyuuhai, and the discarding opponent's nagashi mangan.
Disadvantages
- Opening your hand prevents certain yaku, most importantly riichi.
- Lowers the value of other yaku, such as sanshoku and ittsu.
- The hand must have at least one (non-riichi) yaku in order to win.
- Reduces the amount of tiles you can discard, making it harder to defend.
- Reveals the called tiles to your opponents, possibly allowing them to deduce the value/contents/yaku of that hand.
Yaku
In order for a hand to win, it must have at least one yaku. Closed hands can always call riichi, but open hands need something else. Tanyao and yakuhai are the easiest/most common of the open yaku. Other common candidates include sanshoku doujun, ittsu, honitsu, and toitoi.
A hand with an unconfirmed yaku may end up in atozuke, a state where your hand only has yaku when it wins off a certain tile. For example, a tanyao-only hand with a 78 wait can only win with a 6, since 9 ruins tanyao. This can lead to furiten if you draw the wrong winning tile.
Value
Once a hand reaches 4 han, every han afterwards is reduced in value. So if a hand has 4+ guaranteed han after opening, it's often best to call. At this point, the extra speed from calling is often worth the drop from value.
When a hand is cheap, the opposite effect can occur. If a hand would be 1 han (riichi-only) when closed, or 1 han when opened, going open is often better. This mostly happens with yakuhai pairs - keeping the pair ruins tanyao and pinfu, but pon lets you get 1 han from the yakuhai triplet. Even when considering the score bonuses from riichi (ippatsu, uradora, mentsumo), an open hand's increased speed is often better than the chance for extra points.
Point standings
As with most things in riichi mahjong, a game's specific point standings will alter the need to call. If you need 12k points to escape 4th in all last, you should stay closed with a 4 guaranteed han hand. If you need 1k points in all last, call as soon as a yaku is available.
Tenpai Settlements
After all tiles in the live wall have been drawn, players in noten pay those in tenpai. In addition, depending on the rules, dealers get to keep their seat if they are in tenpai. Therefore, players near the end of the game may wish to call tiles to reach tenpai.
Even if a hand would have no yaku, or would be otherwise impossible to complete, players can still call tiles in order to reach keishiki tenpai ("shaped tenpai"). For the purposes of tenpai settlements, a hand in keishiki tenpai still counts like any other tenpai.