I’m feeling I ought to know more about second line moves. I have generally had the feeling that is best to avoid second line moves, at least in the earlier parts of the game. However, sometimes it seems either necessary or even good to play a second line move very early. To me this is usually because it is a “joseki move” but I’ve no idea really about why you need to resort to a second line move.
As a comparison, I feel comfortable that I understand first line moves - in short, only play them in the endgame, monkey jumps and some tesuji. Similarly, I have a bit of a grasp on third and fourth line moves, territorial and influential and balancing them and that. But second line moves remain mysterious.
Grateful for any thoughts, resources, heuristics, anecdotes, etc about the second line.
I have a few specific positions in mind to understand - when to jump backwards to the second line from the fourth line? should you descend to the second line after playing a knee/armpit hit on the third line? (Otherwise you stand up and get hane on the head of two)
I think trying to make and stick to some ideas like this isn’t very helpful. I mean, vaguely, you don’t want to play second line moves to make territory because you make less per move than a third or fourth line in some situations.
Mostly you make the moves that make sense in a given situation. If one needs a second line move for eyespace, one does it. If one needs to attack a group with a two space or one space extension on the third line, there’s appropriate conditions where a second line placement is quite strong.
Similarly if you have to play a first line move to capture a stone or make a group live you don’t avoid it just because it’s not the endgame (I mean maybe if one avoids playing the move it will be the end of the game )
@teapoweredrobot I find it difficult to say something general about second line moves, without there being exceptions. Usually in the opening and middle game, second line moves are not played without a reason, i.e. you usually don’t see a second line move being played as the first move in a general area. A second line move is usually played due to its relation to other nearby stones.
3-3 invasions feature many examples of second line moves. Reasons include being pressed down to 2nd line, playing a cut / hane on second line for tactical reasons etc.
Edit: I just realised this joseki features multiple second-line and even a first-line move!
As it turns out, the one-point jump K6 has unfortunately little effect to prevent such a connection.
I am a firm believer that one should not study joseki to blindly apply them in the game, but to learn about general good moves in certain configurations. For example we can derive a lot about such a position from a popular corner joseki:
Here A is by far the most popular next move, but B - D are the start of other (old) joseki variations. All of these make it (at least a little bit) difficult for black to connect their stones along the edge of the board.
Here is a possible continuation for B, which I believe I saw somewhere:
I should add that in my previous example, the pro’s move is not the best move according to AI, but is certainly playable for us since it was played by a pro.
Huh, as far as 2nd-line moves go, O2 or B8 look good. But personally I would consider playing a move in the top-right corner, like R17 or O17. R8 or C12 are other good options.
Many years ago, there was an important match between a chinese and a korean kid, 14 or maybe 15 years old and all the top chinese pros were watching. In the very early stage of the game, the korean kid placed the stone at second line on the side. It seemed to be very strange move. The chinese debated and couldn’t agree. After a long pause, the then #1 chinese player Nie Weiping said he figured out that it was a move to give a half point advantage at yose.
Then he sighed and told others: this is scary. no chinese kids can think this far away and counter him.
The main reason to play second line moves in the opening is when they are important as affect the strength and weakness of groups, because their simple territorial value is not usually enough to make them a good move.
When to play second line moves?
Interesting question.
Avoiding in early phase is natural. That is what we taught when we started to play go.
So an early second line move feels unnatural. This is learned behaviour.
Also capturing every stone you ataried is learned behaviour. Later on you learn that it is not necessary to capture every stone. It even can have a negative consequence; you capture one stone and your opponent claims or reinforces a corner.
If a move is automatic it is learned behaviour. You should be suspicious about this type of moves. Also if a not-move on the second line is automatic, it may be interesting to ask yourself: why not play on the second line this early in the game?
This means that you don’t fully understand that joseki.
Moral:
why not play on the second line?
be suspicious of automatic moves
be suspicious of universal laws in go: there are always exceptions to it.
(I mean maybe if one avoids playing the move it will be the end of the game 
