If the angle formed between the intersecting lines are 90o then the two lines are perpendicular. Or If the dot product of the two lines is zero. Or (by Misacek01) In analytical geometry, when the lines are defined by linear functions, you write the functions in the form y = ax + b, where x is the independent variable, y is the dependent variable, a,b are real-valued constants. So, you have two functions for the two lines: y(1) = a(1)x(1) + b(1) ; y(2) = a(2)x(2) + b(2) (the ones and twos are just indices, not powers or multipliers). Then, the two lines are perpendicular if and only if a(1)a(2) = 1