Incenter centers known to the ancient Greeks, and the only one that does not in general lie on the Euler line. It is the first listed center, X(1), in Clark Kimberling's Encyclopedia of Triangle Centers, and the identity element of the multiplicative group of triangle centers. For polygons with more than three sides, the incenter only exists for tangential polygons—those that have an incircle that is tangent to each side of the polygon. In this case the incenter is the center of this circle and is equally distant from all sides. It is a theorem in Euclidean geometry that the three
Isothetic polygon polygons, it is very often implicitly assumed that these polygons have the same alignment (in fact, aligned to the same orthogonal coordinate axes), and hence the term "isothetic polygons" would be less ambiguous. In the context of digital geometry, isothetic polygons are practically axis-parallel and have integer coordinates of their vertices. Isothetic polygon An isothetic polygon is a polygon whose alternate sides belong to two parametric families of straight lines which are pencils of lines with centers at two points (possibly the point at infinity). The most well-known example of isothetic polygons are rectilinear polygons, and the former term is