Text

Si in duas rectas lineas altera recta incidens internos, ad easdemque partes angulos duobus rectis minores faciat, duae illae rectae lineae in infinitum productae sibi mutuo incident ad eas partes, ubi sunt anguli duobis rectis minores.

Translation

If a straight line, when intersected with two straight lines, causes angles formed on the same side to become together smaller than two right angles, then the two straight lines, when extended to infinity, meet on the side on which lie the angles which together are smaller than two right angles.

Author

Euclides

Reference

Evclidis Elementorum libri XV: Accessit liber XVI. Liber I, p. 95

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