The actual configuration or absolute configuration of a molecule is assigned by a set of sequence rules given by three scientists Cahn,Ingold,Prelog referred as CIP system.The sequence rule is given in terms of R and S configurations.
Sequence Rules are as follows:
1)Write in terms of Fischer Projection Formula.
2)identify the atom and assign priority in accordance to their atomic no.
3)if in a molecule different groups are attached to chiral centre through similar atoms,priority order cannot be assigned on the basis of atomic number of the atoms directly attached to chiral centre.In such case we look at the next atoms with higher atomic number present in groups and priority is assigned at the first point of difference in accordance with the atomic number.
4)In case of groups having multiple bonds, for assigning priority order a multiple bond is considered to be bonded to an equivalent number of simmilar atoms through single bond. For example twop carbon attached through a double bond and cosidered as if each carbon is joined through two single covalent bonds to two carbons.