Rulering is the practice of using a ruler as a convenient, hence in some mainly scholarly traditions popular, tool for a corporal punishment, usually on the hands (either outstretched palms or knuckles) or thighs, although sometimes as a spanking (on the buttocks).
In BDSM, where it occurs especially in role-play context, this can be a voluntary, sometimes non-punitive practice.
Generally, solid, heavy wooden or metal rulers are used for these purposes, but lighter, often flexible plastic ones can also be used punitively. Thus the wooden regla (Spanish for ruler) was widely used in schools to punish pupils: the 12" one was applied as a ferule (see palmeta) on the hands, the yard long one (made for drawing in the blackboard; rather like a yardstick) as a rod for spanking.
In the United States, this method of punishment is more common in private than public schools.
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