In this article I derive a concentration measure for markets with multiple vertical segments. I derive the measure using a model of vertical contracting in which upstream and downstream firms bargain bilaterally and may be integrated. The resulting vertical Hirschman-Herfindahl index provides a measure of the degree of distortion in the vertical chain as a result of both the horizontal concentration in a segment and the degree of its vertical integration. This measure distinguishes between the differing competitive impacts of upstream and downstream competition, establishes the relative size of integrated firms in each segment, and provides a quantitative threshold test for vertical mergers.
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