The inverse square law applies to any entity which radiates out from a point in space (see Point Source and Fluence Rate to expand on this concept). With respect to Radiation Protection, the law says if you double your distance from a source of Ionising Radiation you will reduce your exposure by 4. It follows that if you triple your distance from the source, the exposure will reduce to 1/9 of the original value. This concept is one of the cornerstones of radiation protection and is used with the Distance Rule. It is not reliable where the distance between source and measurement position is small compared to the size of the source. Generally, the rule will work where the distance between the source and the calculation point, is at least 10 times the largest dimension of the source.