Why a diagonal can be easy to miss
In Connect Four, pieces fall to the lowest open space in a column. That rule makes a diagonal more than a simple slant: each piece must sit at a different height, and the lower pieces—or other pieces beneath them—must support the pattern. A winning diagonal can therefore be spread across several columns and levels rather than appearing as one obvious row of matching pieces.
Players often scan for horizontal lines because they are easy to see, then check vertical stacks. Diagonals demand a different glance: follow a sloping path and notice how its pieces are staggered. The line may rise from left to right or from right to left.
The last move is the best place to look
A practical way to check for a win is to inspect the piece just played in all four directions: horizontal, vertical, and the two diagonals. Any four-in-a-row completed by that move must pass through it. This turns a broad search of the board into a focused check—and helps catch a diagonal that crosses four rows and four columns.
The same geometry matters before a line is complete. A player can build toward a diagonal, but gravity and the pieces already in each column determine whether the next piece can land where it is needed. A diagonal threat is both a pattern to spot and a landing space to make possible.
