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A through hole, also thru-hole or clearance hole, refers to a hole that is reamed, drilled, milled etc., completely through the material of an object. In other words, a through hole is a hole that goes all the way through something. This is to distinguish it from a blind hole, which does not go all the way through the material of the object.
Taps used for through holes are generally tapered since it will tap faster and the chips will be released when the tap exits the hole.
References
Through hole Wikipedia(Text) CC BY-SA