Illustrated Glossary of Organic Chemistry

Bredt's rule: An empirical observation that a double bond cannot be at a bridgehead carbon, unless the ring that contains the double bond contains at least eight atoms. The rule stems from the observation that elimination to give a double bond always leads away from the bridgehead, unless the ring containing the new double bond is large enough.

E2 elimination of 2-chlorobicyclo[2.2.1]heptane produces bicyclo[2.2.1]hept-2-ene (a Bredt alkene) and not bicyclo[2.2.1]hept-1-ene (an anti-Bredt alkene). Note that the ring containing the double bond contains six carbons. In a ring of this size the alkene cannot be at the bridgehead carbon because such an alkene is too highly strained.