Tests of the Empirical Classification of Horizontal and Vertical IntraIndustry Trade. — Using longitudinal data this paper shows the separation of international trade in (a) inter-industry trade, (b) horizontal intra-industry trade, and (c) vertical intra-industry trade used in the empirical trade literature to be non-stable at the individual product level. The high level of vertical intra-industry trade mentioned in the empirical literature probably covers up many products shifting between e.g. vertical and horizontal intra-industry trade. The paper therefore questions the work of the last decade in the measurement of intra-industry trade.