This refers to the sequential manner in which most goods and services are produced, where a given product uses intermediate products as inputs, which are themselves produced with other intermediate inputs, and so forth. 1 The nature of this production structure has important implications for productivity and the gains from international trade. As has long been acknowledged by trade economists (e.g. Krugman & Venables, 1995), trade in intermediate inputs provides a magnification effect that raises welfare significantly more than trade in final goods alone. This effect is typically captured by assuming a 'roundabout' production structure, which delivers a sizable welfare impact (see Costinot & Rodríguez-Clare, 2014). 2 Although the roundabout assumption is convenient in a number of ways, it abstracts from an additional channel whereby multi-stage production might facilitate gains from trade (GFT), namely through specialisation across production stages or 'vertical specialisation' (VS). Consider, for example, smartphone production. Some popular models are assembled as final products in China, using intermediate inputs like the LCD screen, gyroscope and flash memory sourced from countries like Japan, Switzerland and Korea. In this example and others, the final stage of production (e.g. assembly) is technologically distinct from upstream production stages (e.g. gyroscope production). Vertical specialisation also features prominently in maquiladora trade for Latin American countries and export-processing zones for China and other Asian countries.1 Multi-stage production is also commonly referred to as sequential production.2 Roundabout production assumes that any given good can be used either for final consumption or as an input into production.