Source: CEPII, WTFC (World Trade Flows Characterization) database.

Notes:

Individual data are not available for Belgium and Luxembourg for the whole period.
Illustrations for Belgium or Luxembourg refer to the sum of Belgium and Luxembourg.

Types of trade are distinguished according to Fontagné & Freudenberg (1997) methodology.

One-Way Trade: inter-industry trade.
Two-Way Trade in variety: intra-industry trade of horizontally differentiated products (similar characteristics and unit values).
Two-Way Trade in quality: intra-industry trade of vertically differentiated products (similar characteristics but different unit values).
Unventilated Two-Way Trade: intra-industry trade without information on unit values.

Products are defined at the 6-digit level of the Harmonized System (HS) classification in BACI database and aggregated at the manufacturing sector level according to the CHELEM database classification (BA to GI and KA to KI, see Classifications).

Two-Way versus One-Way Trade

Two partners may export and import the same product. For example, French producers may export cotton men's shirts to Spain, while Spanish producers may export the same category of product to France. There is thus a trade overlap. If the overlap is above a given threshold, then the flow is defined as two-way trade, or intra-industry trade.
Trade at a country-partner-product-year level is considered to be two-way or intra-industry when the value of the minority flow (the smallest value between the export and import flows) represents at least 10% of the majority flow:

If the ratio is below this 10% threshold, the flow in considered one-way.