Page "Perpendicular" Paragraph 12

If two lines ( a and b ) are both perpendicular to a third line ( c ), all of the angles formed along the third line are right angles.

Therefore, in Euclidean geometry, any two lines that are both perpendicular to a third line are parallel to each other, because of the parallel postulate.

Conversely, if one line is perpendicular to a second line, it is also perpendicular to any line parallel to that second line.