Agreed-upon reference materials are required so that reported δ values are comparable among studies. For the sulfur isotope system, δS values are reported on the Vienna-Cañon Diablo Troilite (VCDT) scale. The original CDT scale was based on a sample of the mineral troilite recovered from the Canyon Diablo meteorite at Meteor Crater, Arizona, US. The Cañon Diablo Troilite was assigned a δS value of 0‰. However, troilite from the Canyon Diablo meteorite was later discovered to have variable sulfur isotope composition. As a result, VCDT was established as a hypothetical sulfur isotope reference with a R value of 0.044151 and δS of 0‰, but no physical sample of VCDT exists. Samples are now measured in comparison to International Atomic Energy Agency (IAEA) reference materials, which are well-characterized, lab-prepared compounds with known δS values. A commonly-used IAEA reference material is IAEA-S-1, a silver sulfide reference material with a δS value of −0.30‰ VCDT. S and S abundance can also be measured relative to IAEA reference materials and reported on the VCDT scale. For these isotopes, too, VCDT is established as having δS and δS values of 0‰. The R value of VCDT is 0.007877 and the R value is 0.0002. IAEA-S-1 has a R value of 0.0007878 and a δS value of −0.05‰ VCDT; it has a δS value of −0.6‰ VCDT.