When there are 2 angles on a straight line, i.e.:
So if we know the acute angle is 65°, we can thus determine the obtuse angle as being
These 2 angles, 65° and 115° are called adjacent angles, and are landmarked by a common side. Both the angles have a common side that is “sticking out”, so the angles are thus adjacent.
Angles which add up to 180° are called supplementary angles.
We can thus say that adjacent angles on a straight line are supplementary.