The Teta I5

Credit Goes to blokusstrategy.com
After you have played enough games to get a good grip on what kinds of moves are “good” and/or “bad” in general, you will probably start to recognize patterns that appear frequently inside of games.  Certain moves may start to seem appealing without having to think carefully through why they are good or bad, but just because they instinctively seem “nice”.  Other moves may generate the opposite reaction – they seem to accomplish most of your goals, but something about them seems wrong, and you hesitate to play them.  This is probably because of experience, as you remember similar situations from other games where certain kinds of moves worked, or didn’t.  One such move that often seems to generate a lot of discussion is playing your I5 straight to the wall in order to block your opponent completely out of an area.  One such position can be seen in the image below, but the term “teta I5” (a name coined by toby) applies to any situation in which you play an i5 all the way to one of the edges of the board in order to keep your opponent from advancing past it.
This move is called the “teta I5” after one of the all-time great players, teta, who was not only one of the first players to use it, but is also one of the players who uses it most effectively.
The problem with the teta I5 in general is that it is a double-edged sword – while it can effectively block your opponents out of one of your zones, it also usually blocks you out of a zone of your opponent.  The I5 doesn’t have too many corners to begin with, and if you play it all the way to the wall, 2 of its corners are already blocked (by the wall).  Considering you needed to use one of the corners just to play it in the first place, there can be at most one usable corner left on it after it is played, and this corner is often already blocked (by you more often than not), or at the very least irrelevant either because it doesn’t lead anywhere, or because you already had a corner to play in the same place.
So, should you, or shouldn’t you play such a move, separating you from your opponent?  It depends on the situation, but the rule of thumb is actually that you shouldn’t.  Even if it appears that you have more space “on your side of the line” than your opponent has on the other side, you immediately can’t play on 5 of the open squares along the I5 because you aren’t allowed to touch it – your opponent has no such problem on his side.  In the image below, the left image shows what may appear to be more space for blue than for green, but as you can see on the right, the two zones are almost equal in size.
In this second set up, because the green L5 is already gone, green has no good moves to advance to the right past L5, but red is still able to move to the left (unlike in the first image).  While it is true that green could play 2 moves to try to move beyond the red L5, this makes green waste a turn instead of red.
So, if there are so many reasons why NOT to play the teta I5, then when SHOULD you play it?  Well, for one thing, if neither of the reasons not to play it exist, then go ahead and play it.  In other words, if the zone you are locking your opponent into really is staggeringly smaller than the zone you are locking him out of, then go ahead and seal it up.  Alternatively, if your opponent can’t really benefit from a free turn, usually because the colour you are blocking is dead, or nearly dead already, then go ahead.  The teta I5 is definitely a good tool to have in a colour kill.
Another good reason to play the teta I5 is to “stop the bleeding”.  If your zone is full of holes that your opponent can use, but the i5 will prevent his colour from reaching them, then it might be worth considering.
Or, if you are being attacked by two colours at once, and the i5 is the fastest way to end the attack by one colour so that you can focus on the other, it is worth considering.  Again, we are starting to discuss a more challenging situation, where other factors might come into play, but sometimes playing one final move on one side of the board so you can switch your attention to the other side of the board can be useful.
Finally, one of the best reasons to play the teta I5 is to seal a zone that YOUR other colour is already in, creating a DCCZ.  A good example appears in the image below.