The Principal of Reciprocity states that the time required for seismic energy to travel between two points is independent of the direction it is traveling. In other words, if it requires time T to travel from source to geophone, it will require the same time T if the source and geophone are interchanged. While this may seem trivial, it is an important concept in seismic refraction. The reciprocal time is used in the depth calculation and it is an important QC check in the field.