definition
The path, real or imagined, that the eye follows when perceiving abrupt differences in form, color, or texture or when objects are aligned in a one-dimensional sequence. Usually evident as the edge of shapes or masses in the landscape.
types
Edge the boundary along which two contrasting areas are related and joined together - - the outline of a two-dimensional shape on the land surface.
BUTT EDGE - the simple sharp edge between two contrasting areas.
DIGITATE EDGE - the complex indented edge between two interlocking and contrasting areas.
TRANSITIONAL EDGE - the presence of one or more band(s) connecting two contrasting areas, forming a transitional stage between the two.
DIFFUSE EDGE - soft edge formed by a gradation between two contrasting areas.
Band - contrasting linear form with two roughly parallel edges dividing an area in two.
Silhouette-line - the outline of a mass seen against a backdrop. The skyline is the silhouette-line of the land against the sky.
subelements
Boldness - the visual strength of a line. Smooth, long and sweeping lines are stronger than lines formed by the overlapping of numerous forms, e.g., treetops; edges between strongly contrasting colors, e.g., skylines are bolder than those between similar colors.
Complexity - the degree of simplicity or intricacy of a line, determined by the variety of directions it follows: skylines in ruffed terrain are more complex than on flat plains.
Orientation - the overall relationship of the line to the (horizontal) axis of the landscape or to compass bearings.
suggested vocabulary
Bold/weak Complex/simple Regular/irregular Soft/hard Straight/curving Broken/continuous Diagonal/horizontal/vertical	Concave/Convex Angular/subangular Converging/diverging Jagged/rugged/smooth Parallel/perpendicular Undulating/flowing Geometric/circular/semicircular
dominance
Bold vertical lines which interrupt the skyline tend to dominate weak horizontal lines.
variable effects
Distance - the strength of a line can decrease with distance due to atmospheric haze.
Atmospheric Conditions - clouds, fog, haze, snow can obliterate skylines.
Lighting - frontlighting flattens form and reduces line strength. Often only the skyline remains evident (e.g., mountain ranges). Sidelighting accentuates the silhouette-lines of separate forms. Backlighting blends together forms of equal distance into one outline. In mountain ranges, the ridgelines delineate overlapping flat silhouettes.