We should know the names of these groups. Farthest away is the line that sees, and reports what it sees, but can offer only a limited resistance. This is called the "line of observation" or the "line of outguards." In rear of the line of outguards we have larger groups placed at greater distances. These are called "supports." This is the line that fights. This is the line that makes extensive preparations for fighting (or resisting). It is called the "line of supports" or the "line of resistance."[2] We have one farther and last line of groups which is still larger and occupies still greater distances than the two we have just discussed. This is the safety valve and is called the "reserve," or the "line of reserves." This is the line that gives a sound factor of safety. It will only be called upon in cases of emergency and may therefore generally enjoy a considerable degree of repose. But it and the line of supports combined must have sufficient strength to delay the enemy, in case of a general attack, long enough for our main body to form for battle. Let us look at the line of outguards for further important considerations and distinctions. The enemy's movements and operations should ordinarily be expected where there are for him least difficulties. Large (dangerous) bodies of troops find trouble in marshes, thick forests, steep mountainous country. They avoid these obstacles as much as possible, selecting open country, solid soil, strong bridges, and good roads. Here is where large and strong groups in opposition are necessary. Small and unimportant groups (or no groups at all) should be placed where the enemy's advance is exceptionally difficult. Finally, there will be places between these last two extremes that require just an average amount of attention, that is to say, require groups of medium strength. The groups that are largest and are used at the important places where danger is most expected, are called "Pickets." (These consist of from two squads of eight men each to eight squads.) The least important groups are called "Cossack Posts." (These consist of four men, usually a noncommissioned officer and three privates.) The groups of average importance are called "Sentry Squads." (These consist of eight men, a corporal and seven privates.)