[and the percent diastereoisomer excess as
], where the mole fractions of two diastereoisomers in a mixture or the fractional yields of two diastereoisomers formed in a reaction are 
and 
(
)
The term is not applicable if more than two diastereoisomers are present. Frequently this term is abbreviated to 
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2205