Markovnikov's Rule
In the addition of a protic acid to an unsymmetrical alkene, the hydrogen attaches to the carbon already bearing more hydrogens.
Unit
Dimensionless
In depth
The rule is a consequence of carbocation stability: protonation occurs so as to generate the more substituted and therefore more stable cation, which the nucleophile then captures. Stated more generally, the electrophile adds to give the more stable intermediate.
Examples in the real world
HBr adding to propene gives 2-bromopropane rather than 1-bromopropane.