|Parallel lines are always the same distance apart, and will never touch.
|Perpendicular lines meet or cross to form right (90 degree) angles.
|Straight angles are also known as straight lines, and measure 180 degrees.
|An acute angle measures more than 0 degrees, but less than 90 degrees.
|An obtuse angles measures more than 90 degrees, but less than 180 degrees.
|A right angle measures exactly 90 degrees. Right angles are sometimes called "square corners", because they look like the corners of a square or a rectangle.
|A triangle is a polygon with exactly three sides and three angles.
|An acute triangle is a triangle whose three angles are ALL acute angles (less than 90 degrees).
|A triangle with exactly ONE obtuse angle. (More than one obtuse angle would mean the three angles could not add up to 180 degrees.)
|A triangle with exactly ONE right angle. (More than one right angle would mean the three angles could not add up to 180 degrees.)
|A triangle whose three sides are ALL the same length. (Also called an equiangular triangle, because the three angles would also have equal measures.)
|A triangle with two (or three) sides being equal in length.
|A triangle whose three sides all have different lengths.
|A polygon having exactly four sides and four angles.
|A quadrilateral whose opposite sides are both congruent (equal in length) and parallel. Rectangles, squares, and rhombuses (rhombi) are all special types of parallelograms.
|A parallelogram whose four angles are all right angles.
|A parallelogram whose four sides are all congruent.
|A parallelogram whose four sides are all congruent (making it also a rhombus) and whose four angles are all right angles (making it a rectangle).
|A quadrilateral with EXACTLY ONE pair of parallel sides - the other pair is NOT parallel, so a trapezoid is not a parallelogram.
|A polygon with 5 sides and 5 angles.
|A polygon with 6 sides and 6 angles.