What is a rule and what are the different types of rules available? A rule, sometimes referred to as a ruler or line gauge, is a measurement tool comprised of a straight edge device marked with graduations for measurement. Rules can be made of metal, plastic, or wood. They come in various lengths and widths from which you may choose depending on the nature of your measurement task. The graduations on rules may be etched, scribed, engraved, painted, or printed onto the tool, and depending on the material of the rule will have differing durability. Rules may be flexible or rigid in terms of build, with flexible rules being made to wrap around objects or corners for measurement. The five standard types of rule include: spring-tempered rule, flexible rule, narrow rule, hook rule, and short-length rule.