X-Square Traps in Reversi and the Aims of Book Openings

When playing Reversi against an AI on your smartphone, you might find that an opening phase that seemed even suddenly tightens up in the midgame, leaving you with few moves and surrendering a corner. In many cases, the cause is being forced into a careless X-square move. To avoid being driven into an X-square located diagonally inside the four corners, you must properly understand the true purpose of opening phase book openings.

In this article, we explain book opening structures (such as Rabbit, Tiger, and Cow) and essential midgame theory to avoid being forced into X-square moves, helping intermediate players consistently defeat AI.

The Fatal Risks of an X-Square Move

On a Reversi board, the four corners (a1, h1, a8, h8) are the most powerful positions because a placed disc can never be flipped. The squares adjacent diagonally to those corners—b2, g2, b7, and g7—are known as X-squares, and playing on them during the opening phase or midgame is considered extremely dangerous.

Placing your disc on an X-square makes it easy for your opponent to take a corner along the diagonal line. Giving away a corner lets the opponent establish stable discs along the outer edges one after another, leading to a massive disadvantage.

While beginners often play on an X-square voluntarily, intermediate players losing to an AI usually face a situation where they have run out of other valid moves and are compelled to play there. To avoid such forced moves, you must keep your options open from the opening phase and restrict your opponent's mobility.

Opening Patterns and Aims: Understanding Rabbit, Tiger, and Cow

In Reversi, established sequences known as book openings have been developed through years of study. Book openings named after animals are not meant for mere memorization; they carry the clear objective of preventing the opponent from taking corners while stabilizing your own central discs.

This diagram shows the position up to move 5, e3, in the Rabbit opening, a classic sequence starting with Black's first move f5. In this line, Black and White concentrate their discs on central squares, steering clear of hazardous outer squares. Building thickness in the center like this prevents giving your opponent safe moves later in the midgame while preserving plenty of options for yourself.

Major book openings have the following characteristics and progression variations:

• Rabbit: Follows f5 d6 c5 f4 e3. A solid standard opening where central density is heightened and steady play usually continues for both.
• Tiger: Follows f5 d6 c3 d3 c4. A dynamic variation that shifts direction to challenge the opponent's formation.
• Cow: Follows f5 f6 e6 f4 e3. Develops along a vertical line to solidify the center into a calm position.

The primary aim of applying these book openings is to avoid exposing discs to the outer board unnecessarily. The more your discs spread outward, the easier it is for the opponent to outflank them, ultimately forcing you into an X-square later.

Play in the app

The fastest way to grasp these principles is by playing. Compete against an on-device AI from start to finish, even without an internet connection.

Midgame Disc Control and Endgame Parity

Once the book opening phase ends and the midgame begins, the principle of flipping fewer discs becomes crucial. If you flip a large number of discs early on, your discs become exposed across the board, handing your opponent abundant outflank opportunities. This state is known as high openness.

This diagram illustrates how playing on the X-squares b2, g2, or b7 is dangerous during Black's turn. By selecting central squares with low openness (quiet moves that flip only interior discs), you gradually diminish your opponent's available squares and force them into hazardous X-square moves instead.

In the endgame, parity comes into full effect. When empty squares on the board divide into several regions, choose your moves based on these principles:

• Odd region: Play first into a region with an odd number of empty squares. Since you get to play the final move after your opponent responds, you can seize control of that region.
• Even region: As a rule, do not enter an even region first. If you let the opponent play first, you can claim the final move.

Restricting the opponent's mobility in the midgame to force an X-square move, followed by applying parity in the endgame to take the last moves, forms a solid strategy for defeating the AI.

Practicing with a Smartphone App

Even if you understand the theory, practical training is essential to avoid X-square traps and execute parity on an actual board. For this, reviewing games with a smartphone app provides an ideal study method.

For example, Patent War (Reversi: 80 Days Patent War) is a Reversi app available on the App Store and Google Play. Because its AI processes moves on your device, you can play offline against the AI or complete the full 5-chapter journey mode without a network connection.

After a game, you can rewind the transcript to review AI commentary for free. Reviewing which turn you flipped too many discs on or where you forced an X-square move will help you advance from intermediate to advanced play.

Stabilize the opening phase with book openings, quietly limit disc flips in the midgame, and count squares in each region during the endgame. Keep this approach in mind during matches to build the skill needed to beat the AI.

Play in the app

Patent War is a Reversi app set in a steampunk world. Start playing against the AI and exploring the journey for free.