So this was the XY plane. All these four atoms would be on the same plane. And these four bonds are also on the same plane. Now, whenever we have the two HS on the same side of the plane, or the x and Y, the heavier atoms on the same side of the plane, we have CITs or Z compounds or molecules. In other words, we define C compounds as hydrogens being on the same side. And we define Z compounds as having the heavier groups on the same side.