Stereodescriptors of diastereoisomers with two chirality elements. When the two chirality elements are both R or both S the molecular entity is described as having an l relationship (l = like), when one chirality elements is R and the other S the molecular entity is described as having a u relationship (u = unlike). The notation can be extended to molecules with more than two chirality elements whereby the l, u relationships of the chiral elements are considered in pairs.