A linear pair is a pair of adjacent angles whose noncommon sides are opposite rays.
The two angles together form a straight
angle, so their measures sum to 
, or 
radians. Thus the angles
in a linear pair are supplementary angles.
Conversely, adjacent
angles that are supplementary angles
form a linear pair.
Supplementary angles need not form a linear pair, since they need not be adjacent or share a vertex.
When two intersecting lines form an angle
of measure 
,
either adjacent angle has measure 
when measured in degrees,
or 
when measured in radians,
while its vertical angle has measure 
.
In the illustration, 
and 
are opposite rays,
while 
is the common ray.
The adjacent angles 
and 
form the linear pair.
