Window

Windows are a class of structure where one of your stones, which is connected to one of your edges, is accessible through a tiny gap in the opponents stones. At first glance, they seem to be not all that helpful. They feel far away from the action and often are created through sequences of moves that give the opponent a large amount of territory, which can feel like a losing exchange. However, they are actually quite strong. Their strength comes from the the fact that having a single extra move is usually enough to win the game, and a window can serve to offer you just that by creating a threat the opponent is forced to block, giving you another move.

An Example

After Blue plays 1 and Red plays 2, Blue has a window at the upper *. The red-shaded cells do not have any red stones in them at the moment, but easily could at some point in the future due to the nature of bridges.

A few moves later, Blue realizes that by starting the ladder with 5, Red is going to have to break at some point, giving Blue another window.

The final battle takes place on this board. Red has a lot of stones in Blue's way, but Blue has a couple tiny windows.

Blue starts by strengthening their upper window. Red, presumably, still feels the other direction is more threatening, so opts to block that way instead. Blue gets withing range of their window. Red will now have to block.

Blue is able to use this window to get dangerously close to their other window.

Without the bottom window, Red holds Blue to a ladder they cannot escape

Instead, Blue is able to use the window to their advantage, setting up a double threat to win the game, making use of a trapezoid (shaded).