Elimination reactions remove a leaving group and an adjacent hydrogen to create a new pi bond, and they parallel the substitution mechanisms with beautiful symmetry: E2 is the elimination cousin of SN2 (concerted, bimolecular), and E1 is the cousin of SN1 (stepwise, through a carbocation).

E2 — Elimination, Bimolecular. Like SN2, this is a single concerted step: a strong base removes a proton from the carbon adjacent to the leaving group (the "beta carbon") at the same time the leaving group departs and the double bond forms. The rate depends on both base and substrate. The signature requirement of E2 is geometric: the departing H and the leaving group must be anti-periplanar — that is, positioned 180° apart, on opposite sides of the C–C bond. Only in that alignment can the two bonds' electrons roll smoothly into the forming pi bond. This geometric demand has real structural consequences, especially on cyclohexane rings, where it means both groups must be axial for elimination to occur. E2 is favored by strong bulky bases (like potassium tert-butoxide) and works on all substrate classes but especially tertiary.

E1 — Elimination, Unimolecular. Like SN1, this is two steps through a carbocation: (1) the leaving group leaves first to form a carbocation (slow, rate-determining), then (2) a weak base removes an adjacent proton to form the double bond (fast). The rate depends only on the substrate. E1 shares all the carbocation baggage of SN1: it needs stable (tertiary/allylic/benzylic) carbocations, favors polar protic solvents and weak bases, and is subject to carbocation rearrangements.

For both E1 and E2, when more than one alkene product is possible, you need Zaitsev's rule: elimination favors the more substituted (more stable) alkene — the one with more carbon groups attached to the double-bond carbons — because greater substitution stabilizes a double bond. The major exception is when a bulky base (tert-butoxide) is used in E2: it is too large to reach the more hindered internal proton and instead grabs the most accessible proton, giving the less substituted (Hofmann) product. Recognizing "bulky base → Hofmann product" is a favorite test point.

This sets up the capstone skill of the whole unit: predicting SN1 vs. SN2 vs. E1 vs. E2 for any given combination. The rough decision framework:

Substrate: Methyl/1° → favors SN2 (rarely eliminate). 3° → cannot do SN2; does SN1/E1 (weak nucleophile/base, protic solvent) or E2 (strong base). 2° → the true battleground, sensitive to every other factor.
Reagent: Strong nucleophile that is a weak base → SN2. Strong, bulky base → E2. Strong nucleophile that is also a strong base → SN2 vs. E2 competition (heat favors elimination). Weak nucleophile/base (e.g., just solvent) → SN1/E1.
Heat always favors elimination over substitution (entropy: elimination makes more molecules).

"Take the H"