The farm remains which will be described in the next chapter can be classified rather easily into three main types, (1) short walls or lines of stones across stream channels, forming terraces, (2) linear borders, made of long lines of stones arranged in more or less parallel or concentric lines, and (3) grid borders, a term we have chosen for lines of stones arranged as in the linear borders, but with added transverse rows of stones. The characteristics of these types will be described in this chapter, together with two other important types of remains occurring in association with some of the farms — field houses and boundary markers. It should be realized that most sites show a combination of two or more kinds of field systems, as will be pointed out in describing the sites, although it is convenient to first define each type separately.