4-4 point low approach attach-block joseki
After exchanging for , White is left with two cutting points. Common ways to handle this situation are a emphasising outside influence and b emphasising side territory. c is seen when White is already strong along the side to build a framework.
White has outside influence, Black considerable territory. Black can aim at a later to start an endgame ko, i.e. White can't expect to make much territory along the edge.
After Black can only defend at and can't block at a. If Black blocks, White can play at itself, taking the corner one way or another.
When White is strong on the upper side and does not like to concede a large corner as after a or being kept low as is likely after b is sometimes seen in professional games.
A first interesting point about this joseki concerns . Can White omit this play in order to leave a weakness at the 3-3 point in the corner?
That idea is really in the realm of handicap go: playing the atari is undoubtedly the honte. There is an example from a quickplay game Cho Chikun-Kato Masao 1990-09-09, as here. After Black made strong shape with , and White invaded at once with , making miai of a and b.
In general this would be considered a thin way for White to play.
This Chinese classical joseki frequently occurs in old Chinese games when the marked stone approaches hoshi only after the two stones at the right have been played. Instead of , white a black b might be possible. The overall aim here is probably to settle white quickly.