Huckel’s rule has been expanded to cover fused polycyclic compounds because when these compounds have the requisite number of electrons, they also show unusual stability.
In fused ring systems, only the electrons located on the periphery of the structure are counted when Huckel’s rule is applied.
See Page 28 here
http://www.aptexts.com/books/millerwrm/MillerCH1.pdf (http://www.aptexts.com/books/millerwrm/MillerCH1.pdf)
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