What is Vorlander's rule?

What is Vorlander's rule? I have got it from a book all of a sudden, but without any further information about it; could you please explain it a bit?




Vorlander's rule discuss about the directing powers of substituent present in benzene ring. Taken [from here](https://books.google.co.in/books?id=nYDl4kO4HKUC&pg=PA139&lpg=PA139&dq=vorlander%20rule&source=bl&ots=NGaxf-RNnk&sig=fPRjSNUu3hYkSqB-SPiBA_3Qh_A&hl=en&sa=X&ved=0ahUKEwj2-uS2hcfaAhULq48KHXQ_CXYQ6AEIQzAF#v=onepage&q=vorlander%20rule&f=false):



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> 1. Saturated groups (containing single bonds) direct predominately 2- and 4-positions.
> 2. Unsaturated groups (containing multiple bonds) direct predominately at 3-positions.
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[Here are some examples:](https://books.google.co.in/books?id=Zy6-ahRQ054C&pg=SL15-PA432&lpg=SL15-PA432&dq=vorlander%20rule&source=bl&ots=VCGo16GCW8&sig=5oRylgL0o19qR0pGsZc-V2RmDo0&hl=en&sa=X&ved=0ahUKEwj2-uS2hcfaAhULq48KHXQ_CXYQ6AEIVTAI#v=onepage&q=vorlander%20rule&f=false)


[![enter image description here](https://i.stack.imgur.com/JLtFN.png)](https://i.stack.imgur.com/JLtFN.png)


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