When two angles are said to be supplementary, what do they form?

Two angles are defined as supplementary when the sum of their measures equals 180 degrees. This concept is fundamental in geometry, as it relates to the properties of straight angles and helps in solving various problems involving angle measurements.

When two angles combine to form a straight angle, which measures exactly 180 degrees, they are considered supplementary. For example, if one angle measures 110 degrees, the other must measure 70 degrees for their total to equal 180 degrees. This characteristic of supplementary angles is crucial in various applications within geometry, particularly in the context of polygons and parallel lines intersected by a transversal, where multiple angle relationships come into play.

In contrast, the other options do not reflect the definition of supplementary angles. Angle sums of 270 degrees, 90 degrees, and 360 degrees relate to different angle relationships and are not applicable when discussing supplementary pairs.