Euclid I.13 (Euclid, Elements, Book I, Proposition 13)

Text: https://www.perseus.tufts.edu/hopper/text?doc=urn:cts:greekLit:tlg1799.tlg001.perseus-eng1:1.prop.13

If a straight line set up on a straight line make angles, it will make either two right angles or angles equal to two right angles.

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