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Lesson 1-1
Points, Lines, and
Planes
Building Blocks of Geometry
 Definition:
 The three building blocks of geometry are
points, lines, and planes.
Points
Definition: It is the most basic building
block of geometry. It has no size. It
has only location.
B
A
How to sketch a
dot:
How to label:
C
A
* Use capital printed letters.
* Never label two points with
the same name (in the same sketch).
Lines
Definition: It is straight continuous
arrangement of infinitely many points.
It has infinite length but no thickness.
It extends forever in two directions.
How to sketch:
?
•must have arrows
at both ends
How to label:
?
?
ABC
? ?
(Two methods)
1- small “script” letter
2- any two points on the line
?
n
A
B
C
Possible Names:
AB AC BC
BA CA CB
Planes
Definition: It has length and width but no
thickness. It is a flat surface that extends
infinitely along its length and width.
How to sketch:
horizontal “edges”
M
P
N
* horizontal
* vertical
* other
How to label:
(Two methods)
1- capital “script” letter
2- any three points on the plane
vertical
“edges”
Collinear Points
Definition: Collinear points are points that lie
on the same line. (The line does not
have to be visible.)
A B
collinear
B
C
A
C
non-collinear
Coplanar
Definition: Points on the same plane
Example:
• Points D, E, and F are coplanar.
D
F
E
Line Segment
Definition: Consists of two endpoints and all
points between
How to sketch:
How to name:
A
B
AB or BA
The symbol AB is read as "segment AB".
Congruent Segments
Definition: segments with equal lengths
~ )
( the symbol for congruent is =
Congruent segments can be marked with . . .
Numbers are equal. Objects are congruent.
AB: the distance from A to B ( a number )
AB: the segment AB ( an object )
AB = CD
AB  CD
Incorrect notation: AB  CD
AB = CD
Correct notation:
B
A
C
D
Midpoint
Definition: A point that divides a segment into two
congruent segments.
D
E
If DE  EF , then E is the midpoint of DF.
F
Segment Bisector
Definition: Any object that divides a segment into two
congruent parts which is the bisector of the
F segment.
A
E
AB bisects DF.
AB bisects DF.
B
D
A
A
E
D
B
F
AB bisects DF.
B
D
F
E
Ray
Definition: A ray begins at a point and extends
infinitely in one direction.
How to sketch:
A
R
A
Y
R
RA or RY ( not RAY )
How to name: RA ( not AR )
( the symbol RA is read as “ray RA” )