If only one side chain is present, the carbon atoms of the parent chain are numbered in such a way that the side chain gets the lowest possible number called the locant. For example, the correct locant for the CH3 side chain in structure (I) is 3 and not 4.
If more than one side chains are present then numbering is done so as to give smallest possible number to all the substituents.
3, 4, 6 – Trimethyl octane (correct)
3, 5, 6 – Trimethyl octane (wrong)
