What type of line describes surfaces that are not visible to the observer of a blueprint?

The type of line that describes surfaces not visible to the observer of a blueprint is the hidden line. Hidden lines are represented by dashed lines and indicate edges or features that are obscured from direct view within the object. This visual representation helps convey important details about the design that are not immediately visible, allowing viewers to understand the complete structure and complexity of the object being depicted.

In drafting and technical drawings, using hidden lines is crucial for accurately communicating how various components of a design relate to one another, especially in three-dimensional space. They provide essential context while maintaining clarity in the representation of the blueprint. This use of hidden lines, therefore, is fundamental to effective communication in engineering and architectural drawings.