4. Definitions. The central sliding part of the rule is called the slide, the other part the body. The glass runner is called the indicator and the line on the indicator is referred to as the hairline. The mark associated with the primary number 1 on any scale is called the index of the scale. An examination of the D scale shows that it has two indices, one at the left end and the other at the right end. Two positions on different scales are said to be opposite if, without moving the slide, the hairline may be brought to cover both positions at the same time.