Euclid Book 1 - Proposition 28
Opposite angles equal means straight lines parallel

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To be proved: If a straight line falling on two straight lines makes the exterior angle equal to the interior and opposite angle on the same side, or the sum of the interior angles on the same side equal to two right angles, then the straight lines are parallel to one another.

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There is a line EF crossing lines AB and CD, angle EGB = GHD and BGH + GHD = 180°

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