Zaitsev's Rule
In an elimination reaction, the major product is usually the more highly substituted and therefore more stable alkene.
Unit
Dimensionless
In depth
Alkene stability rises with substitution because of hyperconjugation, and the transition state leading to it is correspondingly lower in energy. The rule is overturned by bulky bases, which cannot reach the more hindered internal hydrogen and give the terminal alkene instead.
Examples in the real world
2-bromobutane eliminating mainly to but-2-ene; potassium tert-butoxide instead giving but-1-ene.