The absolute abundance of an isotope is difficult to measure with accuracy. Therefore, we compare isotopic ratios in a sample with those in a standard resulting in the delta-notation: d(x) = ((Rx - Rst)/Rst) x 103, where d(x) is the delta-value of a sample, Rx and Rst are the isotopic ratios in sample (Rx) & standard (Rst). The d-value is the relative difference in the isotopic ratio of the sample and the standard. It is expressed in part per mille (o/oo); that is why the right-hand side of the equation is multiplied by 103 (1000). Carbon and oxygen data from carbonates are usually referred to the PDB standard (a belemnite, Belemnitella americana, from the Late Cretaceous peedee Formation in South Carolina).