The most obvious gap is a spacing gap. This usually occurs when wings or anchors fail to communicate or must cover a wider distance between the core and flank than they have the manpower to support. Fighters should avoid being separated by more than twice the distance between the lines to help reduce the chances of someone making it through the gap. Communication across the line and shoring up thin spots in the line early will prevent most instances of this type of gap.
Several other types are less apparent to the untrained eye. Stagger gaps and vision gaps are fairly similar, but stagger gaps are the extreme case. A vision gap is one that forms because two adjacent fighters are fixated away from each other, meaning neither of them are watching their middle. A decent fighter can push up quickly and kill one or both of them on their way through the gap. A stagger gap happens when both sides have vision gaps that overlap, causing the lines to stagger if one flank pushes.
The simplest cure for vision gaps is keeping your head on a swivel and using your peripheral vision watch the surrounding area. Staggered gaps give you two options, either exploit it to kill the enemy line, or fall back out of the danger. More often than not, the first side to recognize the danger will be able to utilize it to crush the enemy.
One other gap is the "L" or angled gap. This is more common to see when a larger force has been engaged by two smaller forces from different sides. Each flank of the larger group turns to face the respective enemy, forming an "L". Much like a staggered gap, this leaves a weak point in each line near the corner, especially when the smaller forces are only temporarily aligned to take on the large group. Angled gaps can also be seen near the edges of a kill pocket in extreme cases.
|For this diagram, I assume the two smaller forces combined probably have more fighter in total.|
Defense against angle gaps can be nearly identical to that of a kill pocket, backing away from the corner and extending the line to force the enemy into a strait line. This can be particularly effective when fighting two desperate allies, forcing their flanks to have to worry more about each other. Sometimes, it is entirely possible to have one wing wheel around the corner, towards the enemy. A successful push like this may even force them backward enough to get them into an angled gap. It also might form a staggered gap if the flank has to wheel about their center, rather than around the corner.
Obviously, gaps are dangerous. A good deal of the time, however, they are dangerous to both sides. Good battlefield awareness will let you spot them early and take advantage, or at the very least prevent the enemy from doing so. Just covering the gap can sometimes be enough to deter someone from pressing in against it. You'd be surprised at how often you'll go to kill a gap, only to meet an enemy about to do the same to your line.