a, b, c are the components of the plane normal;

x, y, z are the components of the known point on the plane;

x0, y0, z0 are the point that we test to see if it's on the plane;

why do we say that the plane equation is

ax + by + cz + d = 0where d is basically a "complex" term in the equation ( d = -ax0 - by0 - cy0 ) that needs to be developed to get a result from the equation.

ax + by + cz - ax0 - by0 - cy0 = 0I (think I) understand the equations, and that they are the same, just no why we use the 'd' notation.