The transfer of energy through space is called propagation. In general, radio wave propagation is divided into three broad categories: Sky propagation, Ground propagation, and Line of Sight. It is important to consider the physical environment in which the device will operate in order to determine if the frequency that has been chosen possesses usable propagation characteristics. For example, effective transmission through water might require selection of a frequency in the VLF range, 10 to 100 KHz.