When the sum of the measures of two angles is 90^o, the angles are called ________.

Complementary angles are two angles with a sum of 90^o.

For example, 

In the given figure, the measure of angle ABD  is 60^o  and that of angle DBC is 30^o.  The sum of the measure of both angles is 60^o  + 30^o.  = 90^o. So these angles are called complementary angles. In this case, angle ABD is the complement of angle DBC.