We explain the meaning of a priori voting power and outline how it is measured. We distinguish two intuitive notions as to what voting power means, leading to two approaches to measuring it. One conception, I-power, focuses on a voter's potential influence over the outcome of decisions by a voting body. The second conception, P-power, focuses on voters' payoff, their expected share of a fixed winning ‘prize’. We discuss and rebut some philosophical and pragmatic objections, according to which a priori (as distinct from actual) voting power is worthless or inapplicable.
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