1 If the slopes of two straight lines are k 1 and k2, then the necessary and sufficient conditions for two straight lines to be vertical are K 1 K2 =- 12. Understanding of the conditions for two straight lines to be vertical (1). The necessary and sufficient conditions for the above two straight lines to be vertical only consider the case that both straight lines have slopes. If a straight line has no slope, then two straight lines are perpendicular. The slope of the other line must be zero. (2) Two straight lines L1:A1X+B1Y+C1= 0 are perpendicular to L2: A2X+B2Y+C2 = 0 if and only if: A1A2+B/Kloc.