The Markovnikov rule
Electrophilic addition on a double bond:
When an ionic compound of the H - X type is added to a carbon-carbon double bond, the main product is the one resulting from the most stable carbocation formed during electrophilic attack. or In the addition of a halogen acid to an asymmetric alkene, the hydrogen ion binds to the carbon which carries the most hydrogen atoms.
Example: Large majority: $CH_3-CH=CH_2+HBr\rightarrow CH_3CH^+-CH_3+Br^-\rightarrow CH_3CHBr-CH_3$ Scarce: $CH_3-CH=CH_2+HBr\rightarrow CH_3CH_2-CH_2^+ +Br^-\rightarrow CH_3CH_2-CH_2Br$