I've got a problem with perpendicular planes in Matlab. I start with a plane A and a point P in A. I calculate a plane B perpendicular to A through point P. Equation plane A: -21660x + 1036y + 4669z = 9.22e6 Point P: [129, 46, -1925] If [a,b,c] is the normal vector of plane B, I choose b = 0.5 and c = 1, then calculate a by assuming the dot product of the normal vectors should be zero. This results in a = 2.4 Substituting point P in the formula of B gives me d = 1593. To check if these planes are perpendicular and both cross point P, I plotted them in Matlab. Code (Text): X=0:350; Y=0:300; [X,Y]=meshgrid(X,Y); Z = (-d-a*X-b*Y)/c; mesh(X,Y,Z) i46.tinypic.com/24yu842.jpg Now these planes do not seem perpendicular to me. (They do both cross point P, not shown in the image). Where do I make a mistake?