1. The problem statement, all variables and given/known data Using the Ax+By+Cz+D= 0 equation for planes, where D = -(Axo, Byo, Czo) Find the locus given by the equation if A and B = 0 note: I'm not 100% sure if locus is the right word, english isn't my primary language. 2. Relevant equations Given above. 3. The attempt at a solution Well, if A and B = 0, the equations can be written as Cz+D = 0. Which means z = -D/C = (0x + 0y Czo)/C = zo Since we're in a 3 dimensions space, the way I see it both x and y are unrestricted and can take any values. Does that mean the locus defines a plane parallel to the X and Y axis, at a Z value of Zo?