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Proposition I.2

Construct equal segments by extension

To place at a given point (as an extremity) a straight line equal to a given straight line.

Postulate I.5

Non Parallel Lines

That, if a straight line falling on two straight lines make the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles.

Definition I.23

Parallel Straight Lines

Parallel straight lines are straight lines which, being in the same plane and being produced indefinitely in both directions, do not meet one another in either direction.

Definition I.8

Plane Angle

A plane angle is the inclination to one another of two lines in a plane which meet one another and do not lie in a straight line.