V is for Vienna
Vienna, Austria is the home of the International Atomic Energy Agency (IAEA). The IAEA is the organization which determines the values of and distributes international isotopic reference materials such as SMOW and NBS-19.
When laboratories run analyses, all results are reported with respect to international reference scales. For SIREAL, the scales most often used are V-PDB and V-SMOW. The ‘V’ at the front of the scale name is in reference to a meeting in the 1990’s during which values for SMOW and for PDB were defined at a meeting of the IAEA in Austria.
At that time, a batch of water was prepared for international distribution and its values were defined at 0‰ for both hydrogen δ2H and oxygen δ18O. At that time, a carbonate standard, NBS-19 was defined to have a value of +1.95‰ for δ13C and -2.20‰ for δ18O. There are other isotopic reference materials available from the IAEA, some with defined values, and others that have been measured and the values provided by the IAEA.