Lines
A
line is one of the basic terms in geometry. We may think of a line as a
"straight" line that we might draw with a ruler on a piece of paper,
except that in geometry, a line extends forever in both directions. We write
the name of a line passing through two different points A and B as "line
AB" or as 
, the
two-headed arrow over AB signifying a line passing through points A and B.
Example: The following is a diagram of two lines: line AB
and line HG.
The
arrows signify that the lines drawn extend indefinitely in each direction.
Points
A
point is one of the basic terms in geometry. We may think of a point as a
"dot" on a piece of paper. We identify this point with a number or
letter. A point has no length or width; it just specifies an exact location.
Example: The following is a diagram of points A, B, C, and
Q:
Intersection
The
term intersect is used when lines, rays, line segments or figures meet, that
is, they share a common point. The point they share is called the point of
intersection. We say that these figures intersect.
Example: In the diagram below, line AB and line GH
intersect at point D:
Example: In the diagram below, line 2 intersects the circle
at point P:
Line
Segments
A
line segment is one of the basic terms in geometry. We may think of a line
segment as a "straight" line that we might draw with a ruler on a
piece of paper. A line segment does not extend forever, but has two distinct
endpoints. We write the name of a line segment with endpoints A and B as
"line segment AB" or as 
. Note how
there are no arrow heads on the line over AB such as when we denote a line or a
ray.
Example: The following is a diagram of two line segments:
line segment CD and line segment PN, or simply segment CD and segment PN.
Rays
A
ray is one of the basic terms in geometry. We may think of a ray as a
"straight" line that begins at a certain point and extends forever in
one direction. The point where the ray begins is known as its endpoint. We
write the name of a ray with endpoint A and passing through a point B as
"ray AB" or as 
. Note how
the arrow heads denotes the direction the ray extends in: there is no arrow
head over the endpoint.
Example: The following is a diagram of two rays: ray HG and
ray AB.
Endpoints
An
endpoint is a point used to define a line segment or ray. A line segment has
two endpoints; a ray has one.
Example: The endpoints of line segment DC below are points
D and C, and the endpoint of ray MN is point M below:
Parallel
Lines
Two
lines in the same plane which never intersect are called parallel lines. We say
that two line segments are parallel if the lines that they lie on are parallel.
If line 1 is parallel to line 2, we write this as
line
1 || line 2
When
two line segments DC and AB lie on parallel lines, we write this as
segment
DC || segment AB.
Example: Lines 1 and 2 below are parallel.
