A rule based on empirical studies of ring conformations with a bridge; it states that a double bond cannot occur at the bridgeheads (branching positions) of a bridge unless the rings are large. The rule was discussed by the German organic chemist Julius Bredt (1855–1937) in 1902 and definitively stated by him in 1924. It applies mostly to bridgeheads with carbon–carbon and carbon–nitrogen double bonds. Bredt’s rule is now understood in terms of strain, with small rings not conducing the p-orbitals of the bridgehead atoms to form pi bonds with their nearest atoms (a consideration that does not apply to large rings).