Hane at the Head of One and a Half
Once one has learned to hane at the head of two stones, it becomes a natural instinct to do it even when you have only one supporting stone instead of two. This seems to be a common problem around the 10k level.
If Black has a stone at x, is excellent shape. But without that stone, he is asking for a fight in a situation where he is weaker. He has played into an atekomi and the cutting point at a is crying out to be exploited.
It is quite possible for to be a fine move for tactical reasons based on nearby stones, but it should generally not be your first instinct.
Patrick Taylor: Would playing when d is in place be one of these "tactical reasons" or is it generally better still to extend?
Karl Knechtel: You mean this?
(2 and 3 are a common continuation)
This is effectively a different shape, and is in the majority of cases better than "extending" (really, connecting) directly at a. You would only play a directly if a peep at b in response to would be meaningful, really. (The peep generally forces a, such that is exchanged for b - but that is usually aji keshi on White's part.)
This is the standard response to (which in turn is the standard response to the shoulder hit of Black's first stone). Black's and White's groups are now equally strong, with the same number of liberties.
Author: Dan. I am only 5k, so comments and corrections are most welcome!
Duhii: Hane is correct for some shapes, extension is correct for others... The diagrams are too small, they don't show the rest of the board at all. In an otherwise empty area, extension is a solid, good move. Then again, pushing from behind in that kind of situation looks like a bad move.
golearner: What about hane at the head of two and a half? Is this also weak? (I'm thinking of continuing from the last diagram as follows.) My own answer is yes, this would also be a weaker move than just extending, and it should work for any length of stones, no? I wonder though, if the distance from the edge of the board makes a difference...
TheBigH: I would say that, the longer the two lines, the less the disadvantage. Haneing at the head of five and a half would be less unfavourable than haneing at the head of one and a half.