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Anode heel effect
Beam intensity is greater on the cathode side, so put the thicker end of the anatomy under the cathode.
The rule
The x-ray beam is not equally intense across the field. It is stronger on the cathode side and weaker on the anode side. This is the anode heel effect. You use it by placing the thicker or denser end of the anatomy under the cathode, so the stronger part of the beam goes where more penetration is needed and receptor exposure evens out.