Illustrated Glossary of Organic Chemistry

Gated community rule: In ¹H-NMR spectroscopy, the observation that aryl protons (protons bonded to a benzene ring) can couple only with other aryl protons. Protons not bonded to a benzene ring cannot couple with aryl protons. In addition a proton not bonded to a benzene ring cannot couple with another proton not bonded to a benzene ring if, when tracing the bonds connecting these protons, one must trace through a benzene ring. The same restrictions apply to heteroaryl protons. The name "gated community rule" is derived from the fact that you cannot drive through a gated community unless you are a resident of that community.

		Aryl protons Ha, Hb, and Hc of toluene can all couple (except where equivalent) because they are all part of the 'gated community' (the benzene ring). However the methyl group protons cannot couple with the aryl protons because the methyl group protons are not aryl protons (the methyl group protons are not residents of the benzene ring gated community; their 1H-NMR signal is a singlet).
		The benzylic protons in red cannot couple (their 1H-NMR signals are singlets) because the trace of bonds (in red) between these protons pass through the benzene ring. (You cannot drive through the gated community to get to the other side if you are not a resident of the gated community.)
		The gate and guard at the entrance to an exclusive gated community controls who can enter, and who cannot enter.