Concerted mechanism: Base abstracts beta-H simultaneously as C-X bond breaks. The H-C-C-X dihedral must be 180° (anti-periplanar) for optimal orbital overlap.

Zaitsev (Saytzeff) rule: Non-bulky strong base → more substituted alkene (major). The more substituted alkene is thermodynamically more stable.

Hofmann rule: Bulky strong base (KOtBu, LDA) → less substituted alkene (major). Steric hindrance prevents the base from abstracting the more hindered internal beta-H.

Competition with SN2: For 1° and 2° substrates. Temperature increase favors E2 (entropy-driven — more molecules in products). Bulky base strongly favors E2 over SN2.

Cyclohexane constraint: In chair conformation, anti-periplanar geometry requires H and X to both be axial (trans-diaxial). This can limit which elimination products form.