The following is an adaptation of the book 'Othello: A minute to learn a life time to master' by Brian Rose. Which can be found in the 'Resources' section. Some positions and explanations may differ slightly from the original source due to impossible or unidentifiable positions used, however we tried to remain consistent to the source as much as possible. Many thanks to Brian for allowing us to develop this material.
Chapter 2: Corners & stable discs
Perhaps the most basic strategy in Othello is to take the corners. By the rules of play, it is impossible to flip a disc in a corner, so that if you are able to take a corner, that disc will be yours for the rest of the game. In Diagram 2-1, the disc on h8 must be white at the end of the game: even if Black later moves to both g8 and h7, he can not capture the disc on h8. Moreover, once you have a corner, it is often possible to build a large number of discs that are protected by the corner and can never be flipped. Such discs are called stable discs
In Diagram 2-2, the discs on the bottom row are stable discs, and in Diagram 2- 3, all 21 white discs are stable discs. If this is not obvious to you, then take some time now to convince yourself. Check the 'Put' option on the Diagram 2-3, then try to flip the White stable discs by placing black discs wherever you like. There is simply no way for Black to get “behind” these discs to surround and flip them.
The possibility of building up stable discs usually makes corners very valuable, especially early in the game. If taking corners is that good, then it should be obvious that you usually do not want to give any to your opponent!
Given the rules of the game, the only way for your opponent to take a corner is if you play in one of the squares next to a corner, i.e., the C-squares or X-squares. The X-squares are particularly dangerous, and a move to an X-square early in the game is almost certain to give up the adjacent corner. For example, in Diagram 2-4, White has just moved to the X-square at g7. Although Black can not take the h8 corner immediately, if he can establish even one disc on the c3-f6 diagonal, then Black will be able take the corner.
One possibility is for Black to play b5, capturing the disc on e5, as shown in Diagram 2-5. No matter where White plays, he will not be able to recapture the e5 disc, and Black will be able to take the h8 corner on his next turn. Once black has the corner, all of his discs on row 8 become stable discs, and later in the game he is likely to be able to create stable discs on the right edge as well. In general, the earlier in the game a corner is taken the more valuable it is, as the potential for building up stable discs around the corner is greater. In most cases, moving to an X-square early in the game will prove to be a fatal error, although later in the book we will examine some exceptional circumstances under which early X-square moves are useful.
X - squares
C - squares
While moves to the X-square will usually allow the opponent to take the adjacent corner, for C-squares the degree of danger depends largely on the rest of the squares on the same edge. For example, in Diagrams 2-6, 2-7, and 2-8, Black will quickly lose the h1 corner.
Diagram 2-6: White simply takes the corner on the next move.
Diagram 2-7: White can play h3; Black has no way of capturing the h3 disc, and White will be able to play h1 on his next turn.
Diagram 2-8: Can you see the way that White can capture the h1 corner?
Starting from Diagram 2-8, White should play h3, gaining access to the h1 corner. Even if Black captures the h3 disc by playing h4, as in Diagram 2-9, White still has access to the corner, as shown in Diagram 2-10. As these diagrams suggest, C-squares are often the most dangerous when the adjacent A-square is empty, allowing the opponent to attack the corner by playing into the A-square. We will see many more examples like this in later chapters.
Safe C - squares
While there are many circumstances under which C-squares are bad moves, they are quite often perfectly good moves, and frequently they involve no danger of giving up a corner despite being adjacent to it. Diagrams 2-11, 2-12, and 2-13 all show examples where Black has a good C-square move at h2.
In Diagram 2-11, h2 builds on Black’s stable discs, and offers no prospect of white taking the h1 corner.
In Diagram 2-12, Black must play h2 to prevent White from capturing the h8 corner. Once he does so, he is in no immediate danger of losing a corner.
In Diagram 2-13, black can play h2 and later play another C-square at h7, all with no danger of losing a corner. As these diagrams suggest, the best time to take a C-square is often when you have pieces of your own color in the other squares along the edge.