###
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

###
rectangle

*has 4 right angles

*has 2 pairs of parallel sides

*opposite sides are equal in length

###
rhombus

*has 2 pairs of parallel sides

*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

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