Center line: the line that cuts through the center of a plane, surface, or form.
Contour line: a line on a map joining points of equal height above or below sea level.
Ghost line: a very light and provisional line sketched lightly on paper or screen to help demarcate edges, center lines, surfaces, planes, and contours.
Horizon line: the line at which the earth’s surface and the sky appear to meet.
Hatch line: one of the primary line types used in sketching as infill to suggest shade and shadow.
Isometric projection: a projection method used to represent three-dimensional objects and spaces; in isometric projection, lines parallel remain parallel unlike the converging lines of perspective.
Orthographic: a method of projection in which objects or spaces are depicted or a topographic surface is mapped using parallel lines to project its shape onto a flat plane.
Outline: a line and/or curve used to enclose a flat shape or three-dimensional form; a silhouette is a filled outline.
Picture plane: in perspective, the imaginary plane corresponding to the surface of a picture, perpendicular to the viewer’s line of sight; the picture plane can be thought of as a glass window on which could be sketched the outside world on the opposite side of the glass.
Projection: the process of sketching lines back to vanishing points to recreate the illusion of a three-dimensional image on a flat sheet of paper; an actor is trained to project his or her voice into a room to be heard in the back seats.
Shade: the relative darkness (or coolness) caused by shelter from direct sunlight; in sketching, shading occurs on surfaces directly out of line with the sun or light source.
Tangent: a straight line or plane that touches a curve or curved surface at a specific point while never intersecting with the curve or curved surface; tangent points are transitions between lines and curves that appear seamless.
Vanishing point: the point on the picture plane to which all projection lines converge on the horizon.
Vantage point: the particular angle from which an object or space is viewed; related to the point of view or viewpoint and station point.
Vignette: a frame sketched in such a way as to appear behind a sketched object or product; the vignette works to provide a depth cue in a design sketch.