The Buttock reference article from the English Wikipedia on 24-Apr-2004
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Buttock

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The buttocks or "gluteus maximus" (sometimes just "glutes") are the two masses of muscle and fat covering the posterior aspect of the human (primate) pelvis. They allow humans (primates) to sit upright comfortably without resting weight on their feet, as (for example) cats and dogs do when they sit upright.

They belong to the intimate parts. Exposing the bare buttocks as a protest or for fun is called mooning.

In many cultures, corporal punishment is traditionally delivered on the buttocks (it is then called spanking). This is especially true for lighter punishment and domestic discipline on children (and in some cultures, on wives). Different reasons have been put forward to explain this: