Two planes are coincident when they have the same or parallel normal vectors and their equations are scalar multiples of each other. In this example, the two planes are [i]x[/i] + 2[i]y [/i]+ 3[i]z[/i] = -4 and 2[i]x[/i] + 4[i]y[/i] + 6[i]z[/i] = -8. Notice from the example how they have the same normal vector, and how the coefficients of the second equation are all double the coefficients of the first equation.