Defect in a wall
Here the wall is perfect, while the wall has an obvious defect, at a. There are two extra stones - balanced by the two stones, you might say.
There are numerous types of defects. There are some standard shapes used to exploit them. See
for a number of very well known positions after common joseki.
Some more examples:
How should White play to use the fact that was played on the second line?
Here in fact White at a is a move of high value. But White has a more severe way available.
In this case White has a strong attack with the clamp . It aims at all four circled points.
Black cannot really permit White to cut with atari.
Typically Black will connect at . Then gives Black bad shape.
This is based on variations from 4-4 point low approach, two-space high pincer with side stone.
In fact Black is in trouble immediately if and follow.
This for comparison is the result of a joseki that is out of fashion with the pros.
Here the table point defect at a is more serious than in the previous case, because it is also the centre of three stones and Black can suffer from immediate shortage of liberties. Normally therefore Black responds at b, when White plays at a.
On the other hand, the stones are less likely to become weak, because the sacrifice stones give Black a number of possible kikashi on the upper side. Therefore is less immediately significant.
I've been told by stronger players that each defect in thickness decreases it's value by half. (I read this that a wall with 2 defects is roughly .25 the value of a flawless wall)