What is a 2x4 called in Europe?
A 2x4″ timber (“stud”) in the US is the original dimension before planing — when sold, it is 1/4″ less on each side, making it 1–1/2″ x 3–1/2″. In metric countries, this is sold as a 38 mm x 89 mm timber, literally the same dimension (to the nearest mm).
Imperial expansion.