The rule states that with the addition of a protic acid HX to an asymmetric alkene, the acid hydrogen (H) gets attached to the carbon with more hydrogen substituents, and the halide (X) group gets attached to the carbon with more alkyl substituents.

In short ; When a protic acid HX (X = Cl, Br, I) is added to an asymmetrically substituted alkene, addition of acidic hydrogen takes place at the less substituted carbon atom of the double bond, while halide X is added to the more alkyl substituted carbon atom. In other words, hydrogen is added to the carbon atom with more number of hydrogen atoms attached to it and halide is added to the carbon atom with least number of hydrogen atoms