What is the term for a line used to denote unexposed detail on a blueprint?

The term for a line used to denote unexposed detail on a blueprint is the hidden line. Hidden lines are commonly represented as dashed or dotted lines and are utilized to indicate edges or surfaces that are not visible from the current viewpoint in the drawing. This helps engineers, architects, and machinists understand the full structure or assembly of an object beyond what can be seen or is exposed. By employing hidden lines, it becomes easier to interpret complex components and their relationships within a design, allowing for accurate assembly and manufacturing.

In contrast, visible lines represent the outline of an object that is fully visible, while phantom lines illustrate alternate positions or to show the limits of parts. Center lines are used to indicate symmetry or the center of circles or arcs. Each type of line serves a distinct purpose in technical drawings, contributing to the clarity and functionality of the blueprint.