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I think that this is decided by the number of carbon atoms. The part which has greater the number of carbon atoms, is considered the parent chain. But this fact is not leading to the correct result always. Are there some rules that govern this fact besides the number of carbon atoms.

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    $\begingroup$ Can you give an example in the form of picture $\endgroup$ – Simon-Nail-It Mar 10 '15 at 12:11
  • $\begingroup$ There are some cases where the "number of atoms" rule is not valid. For instance if you have a benzene ring with an ethyl group which in turn has a carboxylic acid group, the main chain would be the ethyl chain and the name of the compound would be 2-Phenylethanoic acid $\endgroup$ – Binary Geek Mar 10 '15 at 13:22
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    $\begingroup$ Related: IUPAC Naming: 2-phenyloctane or 2-octanylbenzene $\endgroup$ – Loong Mar 21 '15 at 15:49
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If you have a simple chain with carbon atoms and an aromatic ring, the priority has the ring, not matter how the chain is long. This rule is NOT valid when you have a functional group: if it is in the linear chain, it has the priority, and the numeration starts from the carbon more near at the functional group, for example $\ce{CH3-CH2-CO-CH2-CH2-CH3}$. The numeration starts from left, so you write hexan-3-one.

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