The standard established for carbon-13 work was the Pee Dee Belemnite (PDB) and was based on a Cretaceous marine fossil, Belemnitella americana, which was from the Peedee Formation in South Carolina. This material had an anomalously high C:C ratio (0.0112372), and was established as δC value of zero. Since the original PDB specimen is no longer available, its C:C ratio can be back-calculated from a widely measured carbonate standard NBS-19, which has a δC value of +1.95‰. The C:C ratio of NBS-19 was reported as . Therefore, one could calculate the C:C ratio of PDB derived from NBS-19 as . Note that this value differs from the widely used PDB C:C ratio of 0.0112372 used in isotope forensics and environmental scientists; this discrepancy was previously attributed by a wikipedia author to a sign error in the interconversion between standards, but no citation was provided. Use of the PDB standard gives most natural material a negative δC. A material with a ratio of 0.010743 for example would have a δC value of −44‰ from .  The standards are used for verifying the accuracy of mass spectroscopy; as isotope studies became more common, the demand for the standard exhausted the supply.  Other standards calibrated to the same ratio, including one known as VPDB (for "Vienna PDB"), have replaced the original.
The C:C ratio for VPDB, which the International Atomic Energy Agency (IAEA) defines as δC value of zero is 0.01123720.