Just as there are two mechanisms of substitution (SN2 and SN1), there are two
mechanisms of elimination (E2 and E1).
E2 mechanism — bimolecular elimination
E1 mechanism — unimolecular elimination
The E2 and E1 mechanisms differ in the timing of bond cleavage and bond formation,
analogous to the SN2 and SN1 mechanisms.
E2 and SN2 reactions have some features in common, as do E1 and SN1 reactions.