Difference between revisions of "Openings on 19 x 19"

Revision as of 20:35, 19 March 2023

19×19 is the most popular of the "large" board sizes. This board size offers a lot of room for strategic freedom (unlike 11×11 or 13×13), but tactics and local play remain highly important.

An average well-played game lasts about 72-90 moves before one side resigns, or 20-25% of the board, though it varies considerably from game to game.

The advice in this guide is heavily influenced by hzy's KataHex bot, the strongest known (and easily superhuman) bot as of March 2023.

Differences from smaller boards

• While corner moves are still good moves, playing near the middle of your opponent's 5th row is often just as good. This starts to become true for boards 18×18 and larger.

• Ladders and ladder escapes are less important. Human games often have long ladders across a side of the board, but it's usually a mistake for the defending side to keep pushing the ladder. Often, it's best for the defender to jump, allowing their opponent to connect in exchange for territory. Here is a common example.

• There is a lot more room to ignore your opponent's threats and play elsewhere in the early opening. Moves are less forcing, and there's a much larger variety of different strategies you can try.

• On smaller boards, the game becomes quite tactical after the opening, and playing well often means playing stones that "work well" with existing stones near the corner. On 19×19, there is room to start a local fight near the middle of the board, relatively far away from existing stones.

Common human mistakes

• Playing too close to your own edge is by far the most common mistake in the opening. There are exceptions where it can be a good idea, like when you're playing a corner move or joseki, or your opponent has intruded heavily into one of your edges, or you're responding to a local tactical situation. However, if your opponent hasn't played near one of your edges, it's almost always a bad idea to play a move closer to that edge than one of your opponent's edges.

General principles

• Corner and edge moves: In the absence of other stones nearby, Red would do well to play in one of the following spots:

This is far from an exhaustive list; many other moves near the middle of Blue's 4th to 6th rows are often just as good. Of course, the presence of other stones even moderately nearby can influence things. KataHex prefers the spots marked (*) especially often.

• If Blue plays too closely to her edge, Red usually has some good local responses. In particular, if Blue plays near the middle of her 4th row, Red can choose one of the following blocks:

KataHex prefers A the most often on a relatively empty board.

• If Blue plays near the middle of her 5th row:

KataHex usually prefers A or E, though B/C/D/F are also common. The move marked (*) is usually less good, because Blue can respond at A.

• If Blue plays near the middle of her 6th row:

Here, all of A/B/C/D are often good choices. The moves marked (*) are usually worse because Blue can respond at A. The move marked (+) is also worse, and Blue usually does well to tenuki.

• If Blue plays close to the center, Red would do well to block at a distance, rather than using an adjacent or near block.

• A well-played game between equally matched players should "use" almost the whole board. In particular, large templates like edge template VI1a rarely matter on 19×19. Many players are templated to play a stone in the middle of their 6th row, because such a stone is connected. However, the opponent has good responses intruding into the template (see above).

• Suppose Red has played the 5-4 opening. It turns out that a decent response by Blue is playing at 3-7 (from Red's perspective), partially due to the threat of Blue 4-4 as a followup. This would imply that, had Blue first played at 3-7 before Red played in the corner, Red should not respond with 5-4, because that would make Blue's 3-7 (which was placed first) unnecessarily effective. Red should instead play a move that works well against Blue's stone. It turns out that the 4-4 corner is such a move. This is an important concept — you don't want to play a move close to your opponent's, if that would make your opponent's stone efficiently placed relative to yours.

• Here's another example. Red accidentally played the 4-5 corner move instead of 5-4. Blue should not play 4-4, because then Red could play 5-4, and he would be in the same position that he would've been, had he played the first move correctly (via the Red 5-4, Blue 4-4, Red 4-5 joseki). Blue essentially let Red out of his mistake. A better move for Blue here is simply to tenuki.

The first move

See Swap_rule#Size_19 for a swap map.

We'll now go through the general strategy of specific first moves. For simplicity, everything will be from Red's point of view, assuming Blue doesn't swap. Unlike the guides for smaller board sizes, we won't think too hard about ladder escapes or switchbacks, and instead we will just mention some brief notes for some selected openings.

Acute corner openings

The stone in the acute corner affects which moves are locally efficient for Red and Blue.

c2

On 13×13, b5 or c6, marked with (*) below, are common Red moves that combine well with c2. On 19×19, these moves are a bit too close to the corner. Playing a bit further along the b5-c6 diagonal, such as A or B below, is often a better move:

b4

Under the right circumstances, Blue c2 (followed by Red tenuki) can be a good local response, though this happens less in the early opening.

e3

e3 is notable because KataHex thinks it's the fairest opening with the swap rule, with KataHex assigning a 49.2% win percentage for Blue, assuming no swap, after 100k visits.

First column openings

If Red starts with a move near the middle of his first column, like a10, a good followup for Red is to play one of the hexes marked A or B, or sometimes C (or both). These moves combine very efficiently with the opening stone to split up Blue's edge. KataHex nearly always plays one of these in the early opening.

If Red plays at A or B, Blue often peeps in Red's bridge, as follows. Red typically responds at (*) or elsewhere, instead of defending the bridge.

a10–a15

Some of the fairer openings in this category are a10, a14, a15. Blue's best response to a10–a15 in the obtuse corner is usually 4-4, but there's no rush to play it:

On 19×19, a15 is weaker than it looks, because the 4-2 obtuse corner, marked (*) below, is less potent for Red than on smaller boards:

a16

a16 is also a relatively fair opening. Blue can play 2-2 obtuse corner like on smaller boards, but it's less clearly the best option. The 4-4 obtuse corner also works, and if Blue instead waits for Red to play 1 as follows, then Blue 2 is a strong response.

Obtuse corner openings

TODO

a19

A common joseki for Red is to play at 1, which is basically the 4-4 opening shifted up one row. Blue often responds at 2, and Red has a couple good responses marked (*):

(todo: move this to a separate "Obtuse corner opening" article?)

My subjective opinion is that this is the most beginner-friendly opening:

• A beginner who opens with Red c2 could accidentally play b3 instead, or alternatively his opponent who wishes to swap Red c2 could implement swap-pieces incorrectly and replace it with Blue c2 instead of b3. Though a19 should technically be swapped to s1 under the swap-pieces convention, it doesn't really matter.

• Aesthetically, a19 retains the "most" symmetry of any fair opening. Beginners who don't want to think about the swap rule could play Hex without swap, where Red must open in an obtuse corner, and such a ruleset would be quite elegant and still balanced, even on large boards.

• For beginners who don't want to learn too much opening theory, "obtuse corner" is easy to remember and a good Schelling point. It's relatively likely that other beginners who look at the swap map and just want to try a random opening will pick a19 or s1.

Third and fourth row openings

TODO