A rule that predicts how substituents will enter the benzene ring. If C₆H₅X is a compound with one substituent in the benzene ring, it will produce the 1,3 (meta) disubstituted derivative if the substance HX can be directly oxidized to HOX. If not, a mixture of 1,2 (ortho) and 1,4 (para) compounds will be formed. The rule was proposed in 1892 by the British chemist Alexander Crum Brown (1838–1922).