Parallel Lines – Notation, Markings, and Angle Clues
Parallel lines are lines that stay exactly the same distance apart forever, no matter how far you extend them, and so they never meet. In diagrams, parallel lines are marked with small matching arrow symbols (› or ››) placed on each line – lines carrying the same number of arrows are parallel to each other.
This page focuses on recognising parallel lines and their notation. When a third line crosses a pair of parallel lines, it creates a rich set of angle relationships – equal and supplementary pairs that let you prove lines are parallel just by comparing angles. That third crossing line is called a transversal, and it's the subject of the very next page in this section.
Recognising and Marking Parallel Lines
Parallel lines are marked with matching arrow symbols in diagrams. If a transversal crossing two lines produces equal corresponding angles, this confirms the two lines are parallel.
Equal corresponding angles are a reliable sign of parallel lines: yes, the lines are parallel.
Real-Life Application
- Railway tracks: the two rails are kept perfectly parallel to carry a train safely.
- Ruled paper: the horizontal lines on lined paper are all parallel to each other.
- Zebra crossings: the painted stripes of a pedestrian crossing are parallel lines.
Key Takeaways
- Parallel lines never meet and stay the same distance apart.
- Matching arrow symbols in a diagram mark lines as parallel.
- Equal corresponding angles across a transversal confirm that two lines are parallel.