In 3D space, we have 3 variables. The standard is to go with x, y, z.
Consider these two equations
x+y+z = 10
x+y+z = 20
Those two equations are inconsistent. Why? We could replace each "x+y+z" with another variable, say the letter w, to get these two corresponding equations:
w = 10
w = 20
At this point, we can clearly see the contradiction. A variable cannot equal two different things at the same time.
In terms of a visual sense, the two original equations plot out parallel planes that never intersect.
