Parallel (geometry)

Two lines in a plane that do not intersect or meet are called parallel lines.

Symbol

The parallel symbol is $Description: \parallel$ . For example, $Description: AB \parallel CD$ indicates that line AB is parallel to line CD.

Lines a and b are parallel.

http://en.wikipedia.org/wiki/Parallel_(geometry)

Transversal line

In geometry, a transversal line is a line that passes through two or more other coplanar lines at different points.

In Euclidean geometry if lines a and b are parallel, and line t intersects lines a and b, then corresponding angles formed by line t and the parallel lines are congruent.

Theorem 9 If a transversal intersects two parallel lines then the alternate interior angles are congruent.

Theorem 10 If a transversal intersects two parallel lines then the corresponding angles are congruent. (Our solved example in mathguru.com uses this concept)

Theorem 11 If a transversal intersects two parallel lines then the alternate exterior angles are congruent.

Theorem 12 If a transversal intersects two parallel lines, then the interior angles on the same side of the transversal are supplementary. (Our solved example in mathguru.com uses this concept)

http://en.wikipedia.org/wiki/Transversal_line

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