If lines are perpendicular to each other then \({m_1} \times {m_2} = - 1\) conversely if \({m_1} \times {m_2} = - 1\) then the lines are perpendicular to each other.
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.