#plane geometry
- Antiparallel line — A straight line that intersects the sides of a given angle in such a way that it forms with them an inverted triangle relative to the symmetry of a classic parallel line. This structural arrangement causes the angle formed by the antiparallel with one side to be perfectly equal to the angle formed by the base line with the other side, having important applications in the study of the antiparallel lines theorem in a triangle.
- Antipedal curve — A plane geometric curve or derived figure obtained as the locus of the feet of perpendiculars drawn from a fixed point (called a pole) onto the normals of a given curve; also, it can define a structural property related to podal curves in higher analytical geometry. This concept is used in studying kinematic properties and complex geometric transformations of lines.