Corner move
During the opening phase of a game of Hex, both player try to occupy strategically important cells. Apart from the center, the four corners are the most strategically important areas of the board (and often they are more important than the center). This is because the corners are where ladders are bound to be heading, and having a suitable ladder escape often makes the crucial difference between winning and losing.
Contents
From Red's point of view
Experience shows that the following cells are the most useful to occupy early in the game. They are shown here from Red's perspective.
Acute corner
In the acute corners, the most important cells for Red to occupy are A or B. Note that these are not on the long diagonal, but slightly further from the red edge. Depending on what is going on on the rest of the board, it is often better to occupy A than B.
- A: this stone is connected to the red edge by edge template IV1a in two different ways. It also serves as a ladder escape for 2nd and 3rd row ladders, as well as for 4th row ladders given enough space or a foldback threat. If A becomes part of a bridge ladder, it will connect to the red edge. Finally, A is close enough to the blue edge to block a good part of it.
- B: this stone is slightly further away from the edges than A. Given enough empty space, it is connected to the red edge by edge template V1a. It helps escape some ladders, though not as easily as A. A bridge ladder forming from B will still connect to the red edge. It also helps to block a large part of the blue edge.
Red should try to occupy A or B in the early game, but not both in the same corner. It is more important to occupy several corners than to have several stones in the same corner.
Obtuse corner
In the obtuse corners, the most important cells for Red to occupy are C or D. Generally, all moves on the short diagonal are good, but C and D turn out to offer the best compromise between connecting to the near edge, blocking the opponent, and offering chances to connect to the far edge.
- C: Given enough space, this stone is connected to the red edge by edge template IV1d. If less space is available, it still creates a 2nd row ladder with good opportunities to pivot and climb. Further, there is the possibility of playing c8 and forming edge template IV2a. A red stone at C can serve as a 2nd-to-4th or 3rd-to-5th row switchback, and it helps to block the nearby blue edge.
- D: This stone is slightly less strong than C towards the nearby red edge, but it is closer to the opposite edge. It does not by itself form an edge template, but it creates a 3rd row ladder, and it also forms a very useful pivoting template. If Red already has C on one side of the board, it is often a good idea to get D on the other.
Red should try to occupy C or D in the early game, but not both in the same corner. It is more important to occupy several corners than to have several stones in the same corner.
From Blue's point of view
The situation from Blue's point of view is exactly symmetric. Note that the acute corner moves for Blue are not the same as Red's, but they are mirrored about the long diagonal:
Larger boards
The above examples were for the 11x11 board. However, the situation on 13x13 or larger boards is completely analogous:
How to reply to a corner move
When a player grabs a corner by playing one of the above corner moves, the opponent is well-advised to not just cede the corner. Rather, the opponent should defend the corner, in an effort to minimize the player's advantage in that area.
There are certain standard and time-tested ways to defend the corners, resulting in a relatively scripted sequence of moves. These standard sequences are called josekis and are described on the page Joseki on this wiki.
Playing any of several possible josekis is usually better than playing no joseki at all, and playing an incomplete or incorrect joseki is usually bad for the player who makes that mistake.
