Organic Synthesis

This shows the addition mechanism starting with a nucleophilic attack by pi bond on Bromine molecule, forming a cyclic bromonium ion bonded to both carbons. The more substituted carbon is then attacked by the nucleophile (H₂O in the above picture), breaking one of the bonds to the bromine, then losing a hydrogen to a bromine ion. NBS can be used as a substitute for bromine.

This follows Markovnikov's rule, with respect to the nucleophile, due to the nucleophile opening the halogen bridge at the more heavily substituted carbon atom. This regioselective to yield a trans configuration.