Kan (2)
Here are some cases where you have material for a kan, but should not declare it.
Your Hand Is Very Bad
Example 1
Draw
Even if this is still early in the hand, you should not kan Example 1.
Once the shape is this bad, your own winning chances are basically hopeless.
Kan increases the number of dora.
In a hand where someone else is more likely to win, giving the whole table extra dora is simply a losing action.
With a hand like this, you can at least cut the honor tiles first,
or even start thinking about folding already and just tsumogiri the .
The Wait Gets Worse
This is one of the most typical cases where you should not kan,
even if you are already in tenpai.
Example 2
Draw Chi 
Pon 
This is a Chinitsu tenpai.
Even if you draw the kan-able , you should quickly tsumogiri it.
Yes, if rinshan and kan-dora cooperate, the final result may reach haneman or more.
But there is no reason to reduce the number of waits just for that extra upside.
Acceptance Decreases
Example 3
Draw
If this were a good-shape one-shanten hand, there would usually be no problem with kan.
But Example 3 is different.
If you split
into
+
you can see that the pinzu block originally still has acceptance from through .
The manzu side is a two-step acceptance anyway, so the best play here is to cut
and take a one-shanten hand with Tanyao guaranteed.
If you later draw or ,
that is the point where you conceal-kan and riichi.
Example 4
Draw
In Example 4, the number of tenpai chances does not directly change even if you kan.
But if a ryanmen completes first, the hand becomes a tanki wait and Pinfu disappears.
And because the hand also still has a possible 789 Sanshoku,
it is better to tsumogiri the tile.
Someone Has Already Riichi'd
If another player has already declared riichi,
then regardless of what your hand looks like, kan is usually NG.
The reasons are:
- the riichi player may gain kan-ura benefits
- if someone has riichi'd, your own winning probability is already lower
Exceptions are:
- you are also in closed tenpai
- you are in a good closed one-shanten hand and the concealed kan still leaves the whole hand excellent
- you are already open, but your wait is very strong and the score increase from kan is huge, such as
3900 -> 7700
plus a few other rare special cases.
You Are Far in First Place
If you are currently first, you should hold back from kan that gives opponents extra opportunities.
That is especially true when your lead is already overwhelming.
Daiminkan from a Closed Hand
Discarded
↓
“Kan!”
This is completely out of the question.
Even if your hand is already open, you still should not daiminkan from a no-ten hand.
And if the hand was still closed to begin with, even less so.
Original Japanese page: http://beginners.biz/naki/naki06.html