The back. The biggest source of heat in a fridge is the warm air that rushes in every time you open the door. Cold air sinks, so it collects at the bottom and, in a fridge freezer, the bottom shelves will be coldest. But in a fridge with an ice-making compartment at the top, it will be the top. Frost-free fridges circulate the air and have much more even temperature distribution.
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