什么是 Perpendicular, parallel？

Two lines are perpendicular when they cross at a right angle (checked with the set square, marked with a little square). Two lines are parallel when they run in the same direction and never cross — like the two rails of a track, always at the same distance from each other.

Two lines are perpendicular when they cross at a right angle; we write $d_1 \perp d_2$ and mark the right angle with a small square. Two lines are parallel when they never meet, however far they are extended; we write $d_1 \parallel d_2$.