Parallel straight angles arising at the intersection.

Two straight lines are called parallel if they do not intersect.
The symbol || is used to denote parallel lines.
a || b - straight line a is parallel to straight line b.

Axiom Through a point that is not lying on a given line, you can draw on a plane not more than one line parallel to this one.



The lines BC and AD intersect the line AB. The line AB with respect to the lines BC and AD is called a secant .
If points C and D lie in the same half-plane from the secant AB, then the angles CBA and DAB are called interior one-sided .



If the points K and M lie in the same half-plane from the cutting AB, then the angles KBA and MAB are called internal crosswise lying .



If a pair of internal crosswise lying angles (angle 1 and angle 2) replace one of the corners with a vertical one (replace angle 2 with angle 3), then the resulting angles are called the corresponding angles (angle 1 and angle 3) of the data lines with a secant.