The City Circle Line or M3 is a loop line of the Copenhagen Metro. It has been claimed by COWI A/S that the City Circle Line is the largest construction project to have taken place in Copenhagen during the last 400 years. The network's total length is 15.5 kilometres (9.6 mi) and has 17 stations. The line opened on 29 September 2019.

Frederiksberg is a part of the Capital Region of Denmark. It is formally an independent municipality, Frederiksberg Municipality, separate from Copenhagen Municipality, but both are a part of the City of Copenhagen. It occupies an area of less than 9 km2 and had a population of 103,192 in 2015.