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plane

an endless flat surface

parallel lines

lines that NEVER intersect

intersecting lines

lines that pass through the same point

perpendicular lines

lines that form right angles (perfect square corners; 90 degrees)

line segment

a part of a line with two endpoints

ray

a part of a line that has one endpoint and continues on forever in one direction

right angle

measures 90 degrees; forms a perfect square corner

acute angle

measures less than a right angle

obtuse angle

measures greater than a right angle, but less than a straight angle

straight angle

forms a straight line

polygon

a closed plane figure made up of line segments

side

a line segment of a polygon

vertex

the point where two sides meet on a polygon

triangle

3-sided polygon

quadrilateral

4-sided polygon

pentagon

5-sided polygon

hexagon

6-sided polygon

octagon

8-sided polygon

equilateral triangle

has 3 equal sides

isosceles triangle

has 2 equal sides

scalene triangle

no equal sides

right triangle

has 1 right angle

acute triangle

has 3 acute angles

obtuse triangle

has 1 obtuse angle

parallelogram

has 2 pairs of parallel sides

square

*has 4 right angles

*has 2 pairs of parallel sides

*all sides are equal

*has 2 pairs of parallel sides

*all sides are equal

rectangle

*has 4 right angles

*has 2 pairs of parallel sides

*opposite sides are equal in length

*has 2 pairs of parallel sides

*opposite sides are equal in length

rhombus

*has 2 pairs of parallel sides

*all sides are equal

*all sides are equal

trapezoid

*has 1 pair of parallel sides

symmetric

when a figure can be folded on a line to form two equal halves that fit on top of each other

line of symmetry

the fold line