Fifth grade was when I first learned the difference between parallel and perpendicular lines. I still remember the worksheet ...

Introduction to electromagnetic waves in electrical engineering. Topics include: concepts of transmission lines, electrostatics and magnetostatics; Maxwell's equations for time-varying fields; ...

Horizontal lines run left to right, parallel to the horizon, with slope 0 and equation y = k. Vertical lines run top to bottom, perpendicular to the horizon, with undefined slope and equation x = k.