Internal dislocation angles are equal and two straight lines are parallel.
The same angle is equal and two straight lines are parallel.
These two lines are parallel and complementary.
Two straight lines are parallel and have equal internal angles.
These two straight lines are parallel and the included angle is equal.
The line segment between two points is the shortest.
The vertical segment is the shortest.
One and only one straight line is perpendicular to the known straight line.
On the same plane, two lines that do not intersect are called parallel lines.
There is one and only one straight line parallel to the known straight line at a point outside the straight line.
If both lines are parallel to the third line, then the two lines are also parallel to each other.
If both lines are perpendicular to the third line, then the two lines are parallel to each other. (inference)