One of four types of edges that may be found in a TIN: regular lines, hard breaklines, soft breaklines, and outside lines. Regular lines define the TIN's basic structure, connecting triangle nodes. Hard breaklines represent features that mark pronounced changes in slope, like roads or rivers. Soft breaklines mark milder changes in slope and sometimes artificial boundaries, such as the border of a study area. Outside lines designate parts of a TIN structure that lie beyond the TIN's zone of interpolation. Every TIN contains regular lines; other line types may or may not be present.