The variation 5.d4 ("ECO" C77), named after George Henry Mackenzie who employed it on a regular basis, is an old line which, according to modern theory, does not promise White any advantage, though is adopted as an alternative to the main variation 5.0-0. The main line continues with 5...exd4 (5... Nxe4 6.0-0 transposes to the Open Defence) 6.0-0 (6.e5 Ne4 is harmless) Be7 (6...Nxe4 transposes to the Riga Variation of the Open Defence) which leads to the Centre Attack ("ECO" C84) of the Closed Defence.