Starting with Q19 is slightly sub-optimal for Black, even under non-Japanese rules.
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Ah, I see what you mean. The opponent could sacrifice two stones to get at least 2 points:
Whereas in this case, everything is urgent the moves are practically mandatory:
Thanks for pointing it out. 
I wouldn’t have reconsidered it otherwise.
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That is not the reason, since the “stones to get at least 2 points” line
sacrifices three stones - not just two - and accordingly gives Black 6 points.
Rather, the main difference is what happens if White chooses ko for the corner .
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Hmm… I am not sure that I can see/find/think that variation.
After blackQ19 , whiteQ18 , blackS19 ,
White has a move that’s sometimes better than R19.
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aaah S18!
I think I’ll send a message to SapporoHeita. He might like to know about all this too! Thanks again for all the info.
It seems a very useful part of learning to determine the status of groups, whether a group is killable or livable in seki (thus safe/“alive”), ko or unconditionally…
(and status problems are useful, as in a real game, everything life-and-death or sekifiable encountered is a status problem to determine what’s possible, rather than a group clearly marked “Black to kill/live” or “Black to make seki for yose reduction”)
I’ve seen some very nice status problems on sekis too, in which it’s useful to know X shape can’t be killed or with determining whether a group is killable, or just yose and thus possibly tenukiable – and a few with very interesting sequences which lead to seki if one wants to avoid ko, etc) though often the seki/ko problems I encounter in the corner (where there are both as possibilities involved) can be more complex to read.
Here’s another classic status problem/life and death problem which has an important branch living in seki.
And here’s a somewhat complex ko/seki one :
Solution
S18 S17 R19 T19 (to avoid B playing there for a ko), Q19 T18 P19 makes a gote seki.
If S18 S17 R19 Q19, T19 makes a ko to kill W.
S18 S17 R19 P19 T18 S19 would be a sente seki for B and less good for W compared to the first variation.)
Of course there are also certain types of capturing races with enough shared liberties which can become seki too.
(for example no eye vs no eye, or eye vs eye of same size)
Or yose situations in which one has to fill in and remove the captured stones to avoid a seki.
For example, like this…
And these can be worth many points if a seki is made, depending on the size of the capturable stones.
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TIL “Sekify” is an actual word
stands for today I learned, things I learned, thought I learned, …?
Would be surprised if this new word had a long life 
It usually means Today I Learned
eye vs no eye ==> no seki.
big eye vs small eye ==> no seki.
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Oups was just noticing that too for uneven eyes, was thinking of forced capturing races, edited.
Interesting! Can you tell me where you found that? Diagram 16 from what?
It looks to me like it’s not a genuine seki: not all of the stones are actually alive. White can safely capture the three black stones in the middle and the two stones on the left edge. After the captures are finished and black throws in again, white has gained some points, and the final position is a more conventional one-small-eye-each seki, yes?
(For the rules lawyers: in a real game, would white need to play out the captures to get the points, or could you finish with this position on the board and claim five prisoners? Does it make a difference whether it’s Japanese, Chinese, AGA or Weird rules? If it came up in a tournament game for me, I think I’d be calling the referee to help with counting!)
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That’s still pretty standard for seki in Japanese rules.
In several cases you gain points in Japanese rules by capturing stones and forcing the opponent to potentially add more stones. It doesn’t make it any less of a seki, it’s just a mistake not to capture capturable stones…
I mean that’s just being slightly finicky. Any capturing race with eyes that comes down to one or two moves you could call a small eye vs small eye seki even if one eye had a big eye to begin with?
It’s not that the naming is chosen to categorise while playing the capturing moves, but rather while trying to analyse in a realistic midgame position?
It’s from Richard Hunters Counting Liberties and Winning Capturing Races
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In a capture race, if the size of the eye is not larger than 3, it should still be considered as a small eye. I remember there is a useful formula 【少年宫围棋班笔记】计算对杀气数的公式【吧闹吧】_百度贴吧.
This corner is slightly bigger, but it’s still possible to create a seki here.
In this rengo, even the other dan player didn’t notice the aji.
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I’m just a beginner (even after maybe 48 years of playing), but it seems to me that on a 9x9 board, all of the sekis I have ever had were characterized by having two neutral points, meaning that almost-alive w and b groups have only two empty points in common.
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That’s not always true, they can have only one empty point in common.
Another one just to show that it can easely fit into a 9x9 board.
When each player gets an eye, it’s quite common to finish into a seki like these.
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Yes! Note the topic of this thread: sekifiable shapes. So far, we have that two points must be in common when there are no eyes in two groups, and one point must be in common when there is one eye in each of two groups. This is what the OP means, I think. Are there any other cases? These should be shown in books that cover dead shapes.