Strategy
Reversi edge strategy
The short answer
Edge discs can only be flipped along the edge or from inside the board, so they are harder to take back than middle discs. They are not permanent until an unbroken run of your own colour reaches a corner. A long edge run that stops next to an empty corner can flip in one move.
Edges are the second most valuable ground on the board and the easiest to misjudge. Half the reason is that they look finished when they are not.
Why an edge disc is only half safe
A disc in the middle of the board can be attacked from eight directions. A disc on an edge can be attacked from five: along the edge in two directions, and from the three directions pointing into the board. The three directions pointing off the board do not exist.
That is a genuine improvement, and it is why edge discs last longer. It is also where the trap is, because "lasts longer" gets read as "cannot be taken".
The unbalanced edge
The dangerous shape is a run of your own discs along an edge that reaches the square next to an empty corner but not the corner itself. Your opponent only needs one move.
Below, Black owns 5 discs in a row on the top edge: b1, c1, d1, e1, f1. The empty squares on that edge are a1, g1, h1, and two of them are corners. Black looks enormously strong up there, with 30 discs to White's 15 overall.
On the very last move of that game White plays a1. It flips 9 discs, and 6 of them are that top edge: b1, c1, d1, e1, f1, g1. The whole row changes hands in one move and the game ends 22 to 42.
Balanced and unbalanced, in plain terms
An edge run is balanced when your opponent cannot play the empty edge square at either end of it, usually because there is more than one empty square between your run and the corner, or because your run already reaches the corner.
It is unbalanced when your run ends on a C-square, the square immediately beside an empty corner. Then the corner move brackets your whole run against a disc your opponent already has further along, and it all flips.
This is the same problem as the one on the X-squares and C-squares page, seen from the edge rather than the corner.
The wedge: the good kind of edge move
A wedge is a disc played into a single empty square with your opponent's discs on both sides of it along the edge. It is one of the few edge moves that is nearly always good.
The reason is mechanical. To flip your wedge along the edge, your opponent would have to place a new disc adjacent to it on that edge and bracket it. Both of those squares are already occupied by their own discs, so there is nowhere to place anything. The wedge can never be flipped along the edge again.
Below, White holds c1 and e1 with d1 empty between them. Black is to move with 5 legal moves, and d1 is one of them. It flips 1 disc, d2, which is the smallest flip on the board.
A wedge also breaks the edge run it lands in, which is often worth more than the disc. White's top edge is no longer a single block that a corner move could take in one go. The computer on this site picks d1 here too.
When to start playing on the edge
- Not in the opening. An early edge disc is a big frontier disc, sitting next to several empty squares, which is exactly what your opponent wants. Mobility explains the cost.
- Wedges any time. If the gap is there and it is legal, take it.
- After you own the corner. Then the edge running away from it is genuinely permanent, one disc at a time. Stable discs covers how far that reaches.
- In the endgame, count. An edge move in the last twelve squares is a disc question and a parity question, not a shape question.
Try it against the computer
Play the computer and watch the top and bottom edges. When it leaves a single gap in its own edge run, take it. When you have four in a row and the corner beside them is empty, count how many moves it takes before that row changes colour.
Next: corner strategy, X-squares and C-squares, stable discs, or back to the strategy guide.
Frequently asked questions
Are edge discs safe in Reversi?
Safer than middle discs, not safe. An edge disc can only be flipped along that edge or from inside the board, which rules out three of the eight directions. It becomes genuinely permanent only when there is an unbroken run of your own colour back to a corner.
What is a wedge in Othello?
A wedge is a disc played into a single empty square between two of your opponent's edge discs. Once it is there, neither neighbour can ever be used to bracket it along the edge, because both squares are already full, so the wedge can never be flipped along that edge.
What is an unbalanced edge?
An edge where your run of discs reaches a C-square next to an empty corner but does not reach the corner itself. It gives your opponent a single move that flips the whole row. A balanced edge either reaches the corner or leaves a gap between your run and the C-square.
When should I start taking edge squares?
After the middle of the game, and preferably once you already own the corner that edge runs into. Early edge discs are large frontier discs that give your opponent extra places to play. Mobility explains why that costs you.