The only London Underground line with a parallel loop, the Northern Line runs from Edgware, Mill Hill East and High Barnet in the north to Morden in the south, branching in the middle into the Bank Branch (east) and the Charing Cross Branch (west). The Bank branch serves the City of London, while the Charing Cross branch serves the City of Westminster. Fabled Mornington Crescent is on this line, north of Euston but only on the Charing Cross branch, which (despite the map) is physically east of the Bank branch at that point. As mentioned above, the Northern Line is coloured black on tube maps.

Edgware               High Barnet
   |                        |
Burnt Oak        Totteridge and Whetstone
   |                        |
Colindale  Mill Hill East  Woodside Park
   |                \       |
Hendon Central       \      |
   |                  \  West Finchley
Brent Cross            \    |
   |                    \   |
   |                     \  |
Golders Green             \ |
   |                       \|
   |                        |
Hampstead           Finchley Central
   |                        |
   |                  East Finchley
Belsize Park              / 
   \                  Highgate
    \                   /
     \               Archway
      \               /
Chalk Farm      Tufnell Park
         \         /
          \     Kentish Town
           \     /
            \   /
             \ /
              V
              |
         Camden Town
              |
             /^\
            /  |
   Mornington  |
    Crescent   |
          |    |
          Euston
          /    \
         /      \-King's Cross St Pancras--Angel-\
        /                                      Old Street
Warren Street                                      \
      |                                            |  
Goodge Street                                  Moorgate
      |                                            |
Tottenham Court Road                               |
      |                                            |
Leicester Square                                 Bank
      |                                            |
  Charing Cross                                    |
      |                                            |
   Embankment                                      |
      |                                      London Bridge
      |                                          /
  Waterloo                                      /
      \                                        /
       \                                      /
        \                                    /
         \                               Borough
          \                                /
           \                              /
            \                            /
             \                          /
              \                        /
               \            Elephant and Castle
                \                    /
                 \                  /
                  \                /
                   \              /
                    \            /
                     \          /
                      \        /
                       \      /
                        \    /
                        |   /
                        |  /
                        | /
                        |/
                        /
                  Kennington
                      /
                    Oval
                    /
                Stockwell
                  /
             Clapham North
                /
          Clapham Common
              /
         Clapham South
           /
       Balham
         /
   Tooting Bec
       /
Tooting Broadway
     /
Colliers Wood
   /
South Wimbledon
/
Morden

Connections with the Victoria Line at Euston, King's Cross St Pancras, Warren Street and Stockwell; the Circle Line at King's Cross St Pancras, Euston for Euston Square, Moorgate, Bank for Monument and Embankment; with the Metropolitan and Hammersmith and City Line at King's Cross St Pancras, Moorgate and Euston for Euston Square; with the Piccadilly Line at King's Cross St Pancras and Leicester Square; with the Central Line at Bank and Tottenham Court Road; with the Waterloo and City Line at Waterloo and Bank; with the Docklands Light Railway at Bank; with the District Line at Bank for Monument and Embankment; and with the Jubilee Line at Waterloo and London Bridge.

The Northern Line Charing Cross Branch is concurrent with the Bakerloo Line from Charing Cross to Waterloo.

Connections to National Rail at Kentish Town, Euston, King's Cross St Pancras for King's Cross and St Pancras, Old Street, Moorgate, Charing Cross, London Bridge, Waterloo, Elephant and Castle Clapham North for Clapham High Street and Balham. International rail connections at Waterloo for Waterloo International.