Why a Reversi corner stays put

Published

This is one subject: a disc flips only when a new disc traps it, and a corner can never be trapped later. The page is the 8 by 8 game this site plays. It is not a catalogue of every Reversi opening name.

1. A move has to trap a line

Black goes first. You may place a disc only on an empty square where at least one straight line of White discs sits between that new disc and a Black disc you already have. Every such line flips. Diagonal lines count. A square that flips nothing is not offered.

The four opening discs sit in the center, two Black and two White. Black’s first placement has to flip one of the White ones. After that, the same test applies on every turn.

2. If nobody can move, the turn is skipped

When no empty square traps a line for the side to move, that side does not get a pass they choose. The turn goes to the other side by itself. The game ends only when both sides have no move, which is often a full board but does not have to be. The side with more discs wins. Equal counts draw.

3. A corner has no later trap

Every other disc can be caught between a new disc and an old one. A corner has no square beyond it. Once a disc is on a corner, no future move can put it between two opposing discs, so its color stays. That is the whole reason a quiet-looking edge move matters more than a big flip in the middle.

Examples on this board

  1. A legal first move. Black plays next to the opening White disc so that White is between the new Black disc and the Black disc already in the center. That one White disc turns Black. A square farther away, with a gap in the line, is not highlighted.
  2. Giving the corner away. You flip several discs along an edge and leave the corner empty beside the disc you just played. White’s reply is the corner, flipping your edge disc. White’s corner disc is still White at the end of the game. The discs you flipped earlier can still change back.
  3. A large flip that is not ahead. A move turns six discs and also leaves White a way into the empty corner on the next turn. The score jumps. The corner, once taken, does not jump back. Counting discs before the corners are decided is how that six-disc move looks better than it is.
  4. A skipped turn. You have discs on the board and no highlighted square. The status moves on to White without you playing. If White then also has no highlight, the game counts the discs and stops.
  5. An empty queue. You ask for a random opponent and wait. After about ten seconds with no other person in this game’s queue, the page says no opponent joined and starts a computer game. That is not a hidden online player. Undo and the local score list apply to that computer game, not to a match the server accepted.

Why it matters

The move list will happily flip a long line. The corner will not offer you a second chance. Players who chase the running count hand over the squares that cannot be flipped, then spend the ending trying to win back discs that were never the stable ones.

The date is the day this explanation was written against the live board, including the public queue and the about-ten-second computer game. A later change to those rules can be told apart from this text.

Closing

Play a square that traps a line, or accept the automatic skip. Treat an empty corner as the square that will not change again. If the queue stays empty, expect the computer after about ten seconds rather than a private opponent. The about page says what the site is, and the privacy page says what the browser and the server store.