Many useful insights can come from comparing the attributes of co-located places. For example, a cluster of cases of lung cancer in a port city might be explained by knowing that asbestos was widely used in the construction of ships in that city in the Second World War. There is also merit in examining occurrences of co-location in point datasets — in many instances the same coordinates are assigned to multiple points in a sample, which only becomes apparent when the attribute table is examined since mapping will obscure the existence of multiple points at the same location. In Section 2.1.5 a distinction was made between cases where multiple attributes are compared for the same set of objects, and cases where the attributes to be compared are attached to distinct, overlapping objects. In the latter case analysis will require the use of special techniques, such as those discussed in Section 2.2.8. Analysts often use the term overlay to refer to the superimposition and analysis of layers of geographic data about the same place. Overlay may require the superimposition of area on area, or line on area, or point on area, or line on line, or point on line, depending on the nature of the objects in the database.