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point names a location line perfectly straight and extends forever in both directions plane perfectly flat surface that extends forever in all directions segment the part of a line between two points ray part of a line that starts at one point and extends forever in one direction angle formed by two rays with a common endpoint called the vertex right angle measures 90º acute angle measures less than 90º obtuse angle measures greater than 90º complementary angles measures that add to 90º supplementary angles measures that add to 180º parallel lines two lines in a plane that never meet perpendicular lines two lines that intersect at 90º angles transversal a line that intersects any two or more other lines Properties of Transversals to Parallel lines the acute angles that are formed are all congruent; the obtuse angles are all congruent; any acute angle is supplementary to any obtuse angle. Triangle Sum Theorem The angle measures of a triangle in a plane add to 180 degrees acute triangle 3 acute angles Right triangle 1 right angle equilateral triangle 3 congruent sides and 3 congruent angles isosceles triangle at least 2 congruent sides and 2 congruent angles scalene triangle no congruent sides and no congruent angles polygon a closed plane figure formed by three or more segments regular polygon a polygon where all sides and angles have equal measure trapezoid exactly 1 pair of parallel sides parallelogram has 2 pairs of parallel sides rectangle has 4 right angles rhombus has 4 congruent sides square 4 congruent sides and 4 right angles slope a number that describes how steep a line is rise vertical change run horizontal change correspondence a way of matching up two sets of objects transformation translations, rotations, reflections translation slides a figure along a line without turning rotation turns the figure around a point called the center of rotation reflection flips the figure across a line to create a mirror image line symmetry a line through an image so that the two sides are mirror images Line of Symmetry the name of the line used in line symmetry tessellation a repeating pattern of plane figures that completely covers a plane with no gaps or overlaps regular tessellation a regular polygon is repeated to fill a plane. The angles at each vertex add to 360º semiregular tessellation two or more regular polygons are repeated to fill the plane and the vertices are all identical. Obtuse triangle Has one obtuse angle