Give an example of `beta` -elimination reaction of alkyl halides.

SN^(1) reaction of alkyl halides leads to

When haloalkanes with beta -hydrogen atom are boiled with alcoholic solution of KOH, they undergo elimination of hyarogen halide resulting in the formation of alkenes. These reactions are called beta -elimination reactions or dehydrohalogenation reactions. These reactions follow Saytzeff's rule. Substitution and elimination reactions often compete with each other. Mostly bases behave as nucleophiles and therefore can engage in substitution or elimination reactions depending upon the alkyl halide and the reaction conditions. In the elimination reactions, the reactivity of alkyl halides follow the sequence

The order of E_(2) elimination for alkyl halide is

Elimination reaction (especially beta - elimination) are as common as the nucleophilic substitution reaction in case of alkyl halides. Specify the reagents used in both cases.