Difference between revisions of "Tom's move"

Revision as of 23:02, 27 December 2021

Introduction

Tom's move is a trick that enables a player to make a connection from a 2nd-and-4th row parallel ladder. It can also be used to connect a 2nd row ladder using a single stone on the 4th row, or to connect a single stone on the 4th row to the edge. Its name originates from Tom Ace (player Tom239), who devised it during a game against dj11, on 15 December 2002 on Playsite. This was not its first use ever, but it is how it came to be known among Hex players on Playsite.

Description

Suppose Red has a 2nd-and-4th row parallel ladder and the amount of space shown here:

Then Red can connect by playing at "*", the so-called "Tom's move".

Usage examples

Connecting a 2nd row ladder using an isolated stone on the 4th row

Red to move and win:

The solution is to push the ladder to 3 and then play Tom's move:

A single stone on the 4th row is connected

Consider a single stone on the 4th row, with the amount of space shown:

Then Red can connect as follows:

Red squeezes through the bottleneck at 2, starts a 2nd row ladder at 4, then plays Tom's move at 6. Note that all of Blue's moves are forced; if Blue plays differently, Red connects outright. This is edge template IV1d.

In a game

Red to move:

Red's d4 group is already connected to the top edge by edge template IV1-a. To connect to the bottom, Red plays as follows:

Now Red is connected by Tom's move. Note that d8 is already connected to Red's group by double threat at c8 and d9.

Why Tom's move is connected

To compute Blue's mustplay region, we consider two red threats:

These leaves only blue moves in the ziggurat.

If Blue plays there other than at a, then Red plays at a.

In that case, Red's 2 connects back via either of the cells marked "*", and since the piece Blue just played is in only one of the x,y,z regions, Red's 2 also connects down via either of the remaining two of those three regions.

Thus Blue's only remaining hope is to play at a.

Red responds like this:

Red's 4 is now connected to the bottom via edge template IV2b, and to Red's main group by double threat at the cells marked "*". Note that 2 and 3 do not actually need to be played; these moves have been included for clarity.

Pushing the 4th row ladder first

Sometimes, there is not enough room to play Tom's move right away, but enough room can be created by first pushing the 4th row ladder. For a typical example, consider the following situation. It is Blue's turn, and Red wants to connect her stone to the bottom edge.

If Red tries to play Tom's move immediately, it doesn't work, because 8 does not connect back to Red's main group.

What Red can do instead is start by pushing the 4th row ladder twice.

Of course this only works if after pushing the ladder, there is still enough room for Tom's move.

It is not actually necessary to push the 2nd row ladder (moves 6–13 can be omitted), but they have been included for clarity.

Note that when Red pushes the 4th row ladder, Blue cannot yield, as this would give Red a ladder escape fork for the below 2nd row ladder. Also, as explained in more detail in the article on parallel ladders, the 4th row ladder must be pushed before the 2nd row ladder has caught up to it. If Red starts by first pushing the 2nd row ladder, then it is too late to push the 4th row ladder.

Variants

Tom's move also works when the hex marked "a" is not empty, provided that "b" connects to Red's main group.

For example, Tom's move works in this situation:

See also

• Edge template IV1d
• 2nd and 4th row parallel ladder escape