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When the lines are parallel:
Interior Angles on the Same Side of the Transversal
(measures are supplementary)
Their "name" is simply a description of where the angles are located.
Interiors on the same side of the transversal make the following possible:
m<1 + m<2 = 180 degrees.
m<3 + m<4 = 180 degrees.
The conclusion is m<1 + m<2 = m<3 + m<4.