Knowing how to use the terrain to advantage to block enemy movement or make your own easier depends on the ability to match the folds in the ground with the way troops move over the ground and how fires relate to the ground and to the enemy. There are five types of terrain classified by relative relief as taught in basic map reading: A hole, a hill, a ridge (finger), valley, and saddle (pass).
These five types of terrain can be illustrated by using the features of one's own hand with the knuckles representing hill tops, the space between as saddles or passes, and the fingers and spaces between the fingers representing ridges and valleys.
Water tends to cut up the slopes into ridges breaking off from ridges creating a complementary complex of dendritic patterns with ridges on the high ground and valleys through which water likes to flow. The appearance of a hand with fingers is easily obliterated but the basic structure remains.
There are five ways to cross the five kinds of relative relief. Three follow the grain by moving along the ridges, or by moving along the valleys, or by going through the saddle or pass. Two go across the grain by running the hill tops which is a bit like ridge running except that the hilltops command the ridges on both sides, and by going cross corridor crosswise the ridges or fingers. This is the most common movement pattern as eventually one gets down the scale to a creek or stream.
In evaluating the application of fire over terrain to enemy movement, the relative slope of the terrain must be taken into consideration. There are three types of slopes: flat or even, convex, and concave. An even slope has the same effect on fires as does flat terrain.
The relation of fires with respect to the ground are classified as either grazing in which the fires graze at a level that engages the enemy for a distance away from the weapons. Plunging fires is when the area which hits the ground, called the beaten zone, is compact and the rounds strike the ground at an angle. In between the two the beaten zone becomes elongated.
Typically, plunging and grazing fires are used in conjuction with grazing fires covering the flat, even or along ridges. Valleys are typically covered with plunging fires from mortars or artillery.
A good commander uses the terrain to force the enemy to advance in column into enfilading fires, and plunging fires to fill in the gaps