Parallel Lines and Angle Pairs
What are Parallel Lines?
Lines are said to be parallel to one another if they are the same distance from each other along their entire lengths. They can be going in opposite directions, or in the same direction; either way they will never meet or intersect - and hence will never form angles with each other.
It is possible however for a third line to cut across two parallel lines, to create angles. We call these lines transverse lines. There are special relationships between the angles created by transverse and parallel lines.
Parallel lines and Angle Pairs
The first rule to remember is that the sum of the angles on either side of the transverse line is 180 degrees.
The second rule to remember is that the internal angle of one intersection will be exactly the same size as the internal angle of the other intersection.
This means that the sum of the internal and external angles at transverse intersections of different parallel lines is also 180 degrees.
Lastly, the internal and external angles on either side of parallel lines intersecting with a transverse line will be exactly the same.