One of the basic observations about Othello is that, once a disc is played in one of the four corner squares, it cannot be flipped. This leads us to a Principle of How to Play Othello: try to play into the corner squares. As with all Othello principles, following it blindly will get you into trouble: here for instance are two positions where Black should definitely not play to the a1 corner.
Figure 1. 59a1?
Figure 2. 53a1?
In Figure 1 above, many beginners might play to a1, taking the corner, rather than b1, letting White take the corner. But the corner is only one of the twenty-nine squares that might change colour between now and the end of the game. Let's try and count which is the right move -- preferably without flipping any pieces. Black currently has 18 discs: if she plays to a1, she will flip all the six pieces from a2 to a7, and all the six pieces on the diagonal from b2 to g7. So after 59a1, Black will have 18+1+6+6=31 discs. Then White will play to b1, flipping just the black disc at b2 (this is the hardest part: figuring out what the board will look like after 59a1 and which discs 60b1 will turn), leaving Black with 30 discs and a loss. If Black plays instead 59b1, she'll flip six discs vertically, five horizontally and five diagonally, leaving her with 18+1+6+5+5=35 discs. 60a1 again only flips the b2 disc, so this time Black wins 34--30. This kind of counting is the most boring part of the game, and it's certainly not the most important, but sometimes it makes the difference between winning and losing. You'll get faster with practice.
What's wrong with 53a1 in Figure 2? Black wins the corner, and it's not only the corner that White can't flip but the entire a-file. However, after 53a1, White can play 54b1, wedging on the North edge. The white piece between the two black sections can't be flipped, so there's nothing Black can do to stop White going to h1, and h8, and a8, not to mention g2 and g7. So Black's a1 is short-sighted, getting one corner and one edge, but letting White get the other three corners and three edges. In fact, after 53a1, White can be even nastier to Black. How? This is Problem 1 -- answers to all problems are at the end of the article. Problem 2 is: what should Black do in Figure 2?
So, is the principle of trying to play to corner squares mistaken? Not really, it's just that there are exceptions. In Figure 1, it's so late in the game that just gathering discs is far more important. In Figure 2, taking a corner has seriously bad consequences for Black elsewhere on the board. But by and large, up until very near the end of the game, it's a good thing to have corners.
Which brings us to formulate our second Principle of Othello. Try not to let your opponent have corners! Of course, (1) blindly following this principle will sometimes lead you astray, (2) it's easier said than done!
The safest way to stop your opponent ever going to a corner is to never play next to a corner yourself. For instance, if there are no black discs on any of a2, b2 or b1, there's no way that White can have a legal move to a1. The squares a2 and b1 (and the six other similar squares next to the other corners) are known as C-squares, while b2 is an X-square (as are b7, g2 and g7). You should think of all these squares as danger squares: playing to one of them might very well let your opponent into the adjacent corner, sooner or later.
Figure 3.
There's a big difference between playing to an X-square and playing to a C-square. Unless there are some unusual features of the position, playing to an X-square in the middle of the game will almost always cost you the corner, as your opponent will be able to find some way to get a disc onto the diagonal. The position opposite is typical: Black has just played to b2, noticing that her opponent can't immediately take the a1 corner. But White can play d8, turning the f6 disc and threatening a1, and if Black replies to f3, turning the disc back, then a White move to g3 turns e5, and there is no way Black can prevent a1. Black shouldn't need to analyse the sequence d8-f3-g3-()-a1 to know that b2 was a bad move; the point is that there is (almost) bound to be a way.
C-squares are much less likely to cost you the corner. Suppose for example you have a black disc at b1. The only way White can use this disc as a stepping stone to the corner is via the North edge, so if you can keep White off the North edge then he won't be able to get to the corner.
Figure 4.
Figure 4 shows some examples of relatively safe edges. The North edge is one of the safest you can have. White can not put a piece on the edge and get to a1 or h1 along the North edge. Note though that, if White gets to one of the corners, he'll have access to the other along the North edge.
The West edge is a little dodgier. If White plays to a3, he threatens to play to a1 next turn. But Black gets to move in between, and if ever White does play a3, Black can play a4, and the edge is all black. Similarly, if White ever plays to a4 (threatening a3, followed by a1), then Black just plays a3, with the same result. The key thing here is that there are just two empty squares on the edge, and if both are played in then the edge is all black. However, note that Black mustn't play to a4: White can then go to a3 and on to a1 next time.
Just the same on the East edge; a White move to h3 is met by h5 and vice versa. The fact that there is a white disc already on the edge isn't too important in itself, as White can't get to a corner using just that disc. Again, Black mustn't play to the West edge first.
The South edge is similar: White can't force his way to a8, as e8 is met by g8. Once again, Black mustn't play to the edge first.
So much for the ``safe'' edges: here are a few which are decidedly unsafe.
Figure 5.
Starting on the West edge this time, White can play to a3, and then to a1 next time; if it is Black's move then there is no way to stop this, as she has no access to a3. Whatever Black did to set up this edge, it was a bad mistake.
The North edge is little better. White might go to f1, when the only way to stop him following with h1 is for Black to play e1. But, after f1-e1, White plays into the one remaining hole at d1, and takes h1 next time. There is a Principle at work here; note that on these two edges there are an odd number of empty squares, and the edges are unsafe, while on the West and North edges of Figure 4 there are an even number of holes, and the edges are safe. Problem 3: if it is Black's move, how can she make the North edge safe?
On the East edge, White's move is h3; he threatens h1, and if Black turns the h3 disc by replying at h4, then White still has access to h1 from the h5 disc. If it is Black's move in the diagram position, then any of h3, h4 or h6 save the edge.
The East and South edges both show the extreme weakness of having just one disc on an edge, in a C-square. White can often play discs to the edge in such a way as to force the win of the corner. The South edge shows this at its most extreme: whatever Black does now, White can force access to a8. If Black plays to c8, then White plays d8; if Black plays to d8 or e8, then White plays c8. Check the details yourself, and you'll see that all of these lead to White having access to a8. Problem 4 is rather more difficult than some of the others: if Black doesn't play on the South edge at all, where should White play to guarantee access to a8?
To sum up, be careful about playing next to the corner. An X-square move is extremely likely to cost you the corner: that might not be disastrous, but you should at least know what's likely to happen to you. C-squares might or might not give up the corner; it depends on the edge position. Be particularly wary of playing your first move on an edge to a C-square.
Magnus Maestro wins in Brussels
Answers to Problems.
1. After 53a1 from Figure 2, White is in no hurry to play b1: he can play all the rest of the moves by starting with 54g2, then (for instance) (P) 55b1 (P) 56h1 (P) 57h8 (P) 58g7 (P) 59a8 (P) 60b7. Black is left with just 9 discs.
2. 53a1 is no good, as we have seen, and 53b1 is even worse, so it must be 53b7! This is a {\sl diagonalisation}: it works because White cannot get a piece on the a8-h1 diagonal, so cannot ever get to a8. In fact, all White can do is repeatedly give away corners to Black: for instance play could go 54g2 55h1 56g7 57h8 (P) 58a1 59b1 -- neither player can go to a8 and Black wins 39--24. Try various other lines and you'll see that White can't do any better.
3. Black can make the North edge safe by playing f1, leaving an even number of holes. Note that e1 doesn't work, as White can then play f1 followed by h1 -- there are an odd number of holes (one) between the C-square disc and the e1 disc.
4. A move to c8 doesn't work, as Black can respond to d8, leaving a safe edge. Similarly e8 can be met by either c8 or d8. So the way to attack the corner is to start with d8! White threatens to play c8 next time, followed by a8, and there is no defence.