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According to the CSM Principle (explained here), the next thing to do after the corners have been taken control of is to try to snatch the sides of the board! This is accomplished by extending along the sides - which basically means placing stones along the sides adjacent to the corners you occupied. Thus the word - extending. You extend your corner positions into the sides. Some examples of such extensions can be found in Fig 1-1.
| Position 1 | Position 2 |
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Now - the question is - where to extend? Which point on the empty side to choose? Basically, the question boils down to this: How far from the corner do I want to play? Or, alternatively - How close to the corner do I want to play? Like with many things at Go, a balance must be struck here.
Lets first consider the problem of "too far". Ideally - you want your stones as far apart as possible, since this way they take control of the largest area - and are thus the most efficient in terms of controlling space. However - If you place your stone too far apart (or, in this case - too far from the corner), there might be enough room for your opponent to "wedge" his stones in-between and thus separate your stones (or, in this case - separate the extension stone from the corner). So "too far" is not good because it is too lose and leaves too much weakness behind for the opponent to exploit at some point. If your extension is too lose, you are said to have over-extended.
So, how about the problem of "too close"? Ideally - you want your stones stuck close together because this way they are connected (or easily connected) and thus can support each other. Such stones are strong and the enemy can have a hard time attacking them. The are very efficient in terms of connection and strength (or thickness, as it is called in Go). However - If you place your stones too close to each other (or, in this case, too close to the corner), they will not be able to take control of a large enough area, and you may lose the game because you fail to accumulate enough territory. So "too close" is also not good, because it is too tight and fails to take control of enough area. When you extend too tight, your move is said to be slow (in terms of controlling area).
Too loose - no good! Too tight - no good! Like with the question of how high or how low to play in the corner (look here) - a compromise must be reached. Each extension has its advantages, and its disadvantages - and it is up to you to chose which of them are important in a given position and which are not. Is it more important to play tight and safe and slow, or can you afford to play loose and fast and take control of larger areas? Questions of balance and trade-off like this are very common in Go - in fact, you have to grapple with such issues before almost every move - but they are easier to answer the more experience you have.
This thing must certainly seem to you like a whole bunch of creepy mumbo-jumbo by now! Hehe... Do not worry - there are some principles which can help you decide where to extend. First of all - lets look at the most common extensions!
| Position 1 | Position 2 | Position 3 |
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The largest extension on a 19x19 board which still retains some connection between the stones is a 3-point extension. It means there are 3 empty points in-between the stones. (see Position 1, Fig 2-1) You will see some 4-point and even 5-point "extensions" - but such moves, while common, are more like putting your presence in an area than actual extensions from the base stone. The 3-point extension is not very solid and the enemy may try to split such stones (or invade the area). Not to worry - such invasions are an integral part of Go, and you cannot escape them! The thing is that in a 3-point extension your stones are close enough to give each other support in the case of such invasion (while larger extensions offer this advantage in much lesser degree, if at all). Such invasion is shown in Position 2 of Fig 2-1 and one way of dealing with it is shown in Position 3. Do not worry too much about the meaning of each move in Position 3 - this is just an illustration.
| Position 1 | Position 2 |
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The second extension is the 2-point extension - which means that there are 2 empty points in-between the stones - you can see it in Position 1 of Fig 2-2. It is very difficult for the enemy to attack such extension - and the reason is that these stones cannot be separated (or at least not with some more enemy support in surrounding area). What happens if he tries to invade is shown in Position 2 - as you see, the invasion is not very successful.
| Position 1 | Position 2 |
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The next extension down the line is a 1-point extension - which means that there is one empty point in-between the stones! This is the tightest extension, and there is no way the enemy can invade it (or at least - not without some serious support from other stones). This is the tightest and the safest of the playable extensions - and also the one that controls the least territory. It is shown in Position 1 of Fig 2-3. What happens if the opponent tries to invade is shown in Position 2.
The 0-point extension (hehe) - or just attaching one stone to the other - also has its place and is often played, but it is not really considered an extension, and usually it is played to strengthen your stones rather than to take control of more area.
On 9x9 board the most common extension you play is the 1-point extension. 2-point extension is sometimes played too, but 3-point extension simply does not exist - or, rather, it does exist but it spans the whole board, and so it is thought of in terms of occupying adjacent corners rather than as an extension along the side.
3-point extension is very popular on a 19x19 board though.
After plowing through all this above material about extension - you may notice that there is one question which remains unanswered yet - namely Which side should you choose to extend along? This is a very complex issue - one which depends as much on personal taste as on some deeper Go theory. Nevertheless - some ideas about this are given here, just to get you started. Also, some more ideas about where to extend and which size of extension to choose are given on the next page.