
If two parallel lines are intersected by a transversal
then alternate interior angles are congruent. 
In a plane if two lines are both perpendicualar to a third line,
then the two lines are parallel. 
If two parallel lines are intersected by a transversal
then sameside interior angles are supplementary. 
If two lines are both parallel to a third line,
then the two lines are parallel. 
If two lines are intersected by a tranversal and alternate interior
angles are congruent,
then the lines are parallel. 
If two parallel lines are intersected by a third line,
then the lines of intersection are parallel. 
If two lines are intersected by a tranversal and sameside interior
angles are supplementary
then the lines are parallel. 
If two planes are both parallel to a third plane,
then the two planes are parallel. 



