1) We can number carbon atoms from left to right or from right to left so that sum of the numbers indicating the positions of substituents and functional groups should be minimum possible.

2) The functional group carbon should be given the lowest number even if it does not obey Rule (1).

3) The carbon atoms of the chain terminating functional group say –CHO, or –COOH groups should be given always number ‘1’ even if it does not obey Rules (1) & (2).

Longest Chain rule : Select the longest continuous chain of Carbon atom which is known as parent chain or principle chain. All the other Carbon atoms constitute in the chain are branched chains or side chains. 1 2 3 4 5 CH3 -- CH -- CH2 -- CH -- CH3 CH3 CH3