A Network Code is a 1 or 2 character identifier that is assigned by the International Federation of Digital Seismograph Networks (FDSN) to provide uniqueness to seismological data streams. Permanent networks are assigned a Network Code that never changes. Temporary networks are assigned a Network Code using the following convention: the Network Code begins with the letter X, Y or Z; or a number 1 through 9 and is then followed by another letter. These codes are only valid for specific years and can be recycled in subsequent years for another temporary experiment.