The Greenwich Meridian passes through London, and there you can read the inscription: London, Saigon, Hanoi

London - Saigon - Hanoi

London - Saigon - Hanoi

Greenwich meridian

The prime meridian is used to measure the meridian latitudes. Go through London and from there, you realize that cities like Hanoi and Saigon they are just one step, hence London, Saigon, Hanoi.

London to Hanoi and Saigon
Hanoi, Saigon within walking distance. London to Hanoi. London to Saigon

London to Madrid. Madrid to Saigon. Madrid to Hanoi
London to Madrid

This is the exact location of the Greenwich Meridian:

Greenwich meridian