Line 6 of the Madrid Metro opened originally between Cuatro Caminos and Pacifico in 1979. This is one of two circular lines in Madrid, but unlike Line 12, it did not open as a full circle. The circle was completed in 1995, taking four stages from its original opening.
First on 7 May 1981, the line was extended from Pacifico to Oporto, then on 1 June 1983, the line was extended from Oporto to Laguna. Thirdly the line was extended from Cuatro Caminos to Ciudad Universitaria serving Madrid's Complutense university on 13 January 1987, and lastly the line was extended from Ciudad Universitaria to Laguna on 10 May 1995, completing the circle. Arganzuela-Planetario station opened on 26 January 2007 between Legazpi and Méndez Álvaro. This station serves Madrid's Planetarium and IMAX theatre.
Line 6 is one of the busiest lines on the network, so to ease congestion on the busiest stations, Madrid adopted the "Barcelona method". This means that at some stations there are two side platforms, and an island platform. This was also used on Line 5, but only two station preserves the original layout (Campamento and Carabanchel).
Line 6 uses 6-car trains of mostly class 8400, however the line still has class 5000's on the line.