Prior to the unification of branding in 1908, each individual operating company had its own branding and logo. Thus there was no definitive 'London Underground' logo during this period.
In 1908, the individual companies operating the London Underground at the time agreed upon a unified branding design, the central element being the then new 'Underground' name and logo.
Due to the technical restrictions of the time, this roundel could be seen in various dimensions depending on its application.
The letters LPTB (London Passenger Transport Board) were added to the roundel in 1933.
The words 'London Transport' were added onto the roundel in 1933.
In 1949 "London Transport" was removed from the logo, and the name was changed to "RAILWAYS".
In 1951, the outlining was removed.
In 1969, the letters were changed to be uniform in height.
In 1986, the traditional red and blue roundel was brought back into use. This time it has been modified so the circle and bar strokes are the same width. The colours have also been brightened.