Purchasing power parities
"The property whereby the direct PPP (Purchasing power parity) between any two Member States (countries) yields the same result as an indirect comparison via any other Member State (country)." Article 3(n) PPP Regulation.
For example, in the case of the three countries A, B and C, the ratio of the PPP between A and B and the PPP between C and B is equal to the PPP between countries A and C as follows: PPPA/C = PPPA/B / PPPC/B.
Eurostat, Organization for Economic Cooperation and Development (OECD), "Eurostat-OECD Methodological Manual on Purchasing Power Parities", Publications Office of the European Union, Luxembourg, 2012