Prehistory and origin of Stockholm
According to the Chronicle of Eric, written in the 1320s, Stockholm was founded by Birger Jarl around 1250 as a lock to the Lake Mälaren region ("laas fore then sio", "lock before that lake") in order to prevent pirates from reaching the seven cities and nineteen parishes around it. Another medieval source ("Visbyannalerna"), however, claims the city was founded in 1187 following a pagan pillaged the city of Sigtuna, and as there was an "Earl" named Birger around at that time too, the disputed origin of the city are likely to remain obscure and some historians choose a diplomatic interpretation saying there was some sort of fortification around by the mouth of lake Mälaren when the city was founded during the second half of the 13th century. As Snorri mentions no city in his account but some sort of fortification called a "kastali" (in various manuscripts curiously said to be located east and west of "Stocksund"), it has generally been assumed this fortification eventually developed into the castle Tre Kronor located where still is the Stockholm Palace.
Storstockholms Lokaltrafik
SL has its origins in AB Stockholms Spårvägar (SS), a city-owned public transit company which started in 1915 by the City of Stockholm with the aim to deprivatize the two separate private tramway networks into one more efficient company. SS would in the late 1920s also acquire private motorbus companies. The first part of the Stockholm Metro was opened in 1950. SS was renamed to SL in January 1967 when the metro, local train, and bus operations in Stockholm County were merged into a single organisation under the supervision of Stockholm County Council. The different mass transit systems within the County had until then been run by different organisations, Statens Järnvägar, private companies and companies owned by the local municipalities.