The first plans for a defencive line on the Karelian Isthmus were made after the Finnish Civil War by Carl Gustaf Mannerheim, but they were ignored when Mannerheim resigned after the war.
The line was constructed during the 1920s and 1930s. It runs from the coast of the Gulf of Finland through Summa to the Vuoksi river and ends at Taipale. It consists of about two hundred machine gun positions made from concrete. The area around Summa was the most heavily fortified because it was thought to be the most vulnerable position. The first bunkers were built between 1921 and 1924. A second phase began in 1932, but was interrupted by the Winter War.
The name Mannerheim Line was spread by foreign journalists and it was supposedly coined by Jorma Gallen-Kallela.
In the Winter War the Line halted the Soviet advance for two months.