Two lines, vectors, planes, etc., are said to be perpendicular if they meet at a right angle. In 
, two vectors 
and 
are perpendicular if their dot
product
 |
(1)
|
In 
, a line with
slope 
is perpendicular to a line
with slope 
. Perpendicular objects are sometimes said to be "orthogonal."
In the above figure, the line segment 
is perpendicular to the line
segment 
.
This relationship is commonly denoted with a small square
at the vertex where perpendicular objects meet, as shown above, and is denoted 
.
Two trilinear lines
 |  |  |
(2)
|
 |  |  |
(3)
|
are perpendicular if
 |
(4)
|
(Kimberling 1998, p. 29).
