The issue of numbering London's bus routes is a thorny one.
The system is not an entirely logical enterprise — being one which has evolved and mutated over many years.
Like our fair metropolis, some of the best bits of the haphazard scheme (like historic precedent) have been preserved, while certain useless elements have been jettisoned (laters, suffixes).
So, when did London first get numbered bus routes?
London's bus routes first started being numbered in 1906.
Before then, in Victorian times, passengers would recognise their bus by its distinctive coloured livery and line name, much like we do today with our tube lines. Buses had the two termini painted on the sides to indicate their route.
Then a George Samuel Dicks from the London Motor Omnibus Company noticed the line name 'Vanguard' was very popular, and decided to name all his lines Vanguard, adding a number for the company's five routes 1 to 5.
It caught on. Other operators realised numbers were easier for the public to remember, and so the practice spread.
For another 18 years, there was no universal system in place; independent operators simply chose the numbers themselves.
Imposing order on London's buses: the Bassom Scheme
In 1924 the London Traffic Act was introduced; one of its features was a numbering scheme for London's buses.
The Met was then responsible for allocating route numbers to buses. The system was known as the Bassom Scheme, after the then-chief constable of the Metropolitan Police responsible, AE Bassom.
Under the scheme, double-decker bus routes were numbered 1 to 199; single-decker routes from 200; and trolleybuses from 500.
This system was revised in 1934 when London Transport was formed, and the task of numbering routes returned to the people who worked in the transport industry.
But historic numbers for routes stuck, even if the buses serving them were changed from single to double-decker, or vice versa.
What does the bus numbering system look like today?
Today, numbers 1 to 599 are for your everyday day routes; school day services are numbered 600 to 699; 700 to 899 are for regional and national coach services.
As this fascinating email explains, TfL tries to work sympathetically with the transport solutions of the past when numbering new bus routes in London.
As London has grown and evolved, so more routes have been added; thus there's not really any chronological — or much geographical — order to the numbers.
Blogger Mark Hadfield asked TfL about the number 55; in his email reply, the anonymous TfL employee explains that the 55 came from tram 55, replaced by trolleybus 55, that had run along Old Street to Hackney, and then evolved into today's bus route 55, from Oxford Circus to Leyton, along its predecessors route.
Similarly, the 207's history can be traced back to what was once Tram 7 between Uxbridge and Shepherd's Bush Green. That tram was replaced by trolleybus 607 in 1936 (the '60-' coming from the Bassom Scheme), and subsequently by 'motor' bus 207 in 1960.
Today's 207 is the fifth busiest route in London, and runs from Southall and White City bus station.
"When we introduce a new route — or make alterations to an existing route by splitting it — the last digit or digits of the historic 'parent' route are used wherever possible, so that passengers might associate the incoming route with its predecessor," says that anonymous TfL worker.
So, when the 53 between Plumstead and Lambeth North was split into two sections in 2003, the new part became 453, between Deptford Bridge and Marylebone.
What about those bus route letter prefixes?
Prefixes first came into use in 1968, under London Transport. Some prefixes have straightforward meanings: C stands for Central; X stands for Express routes; N denotes a night bus.
With others, the prefix letter designates the place around which the route clusters.
So P for Peckham for routes P4, P5, and P13; E for Ealing in series E1 to E11. Then there's the RV1 — a nod to the river it hugs along its route; and the G in south London's G1 is for St George's Hospital.
Do you remember suffixes on bus routes?
Bus route numbers with letter suffixes are no more, but they were in use for many years.
Letter suffixes started being used in the 1920s, to denote branches of different routes.
But suffixes were gradually abolished over the years. The last suffixed route in London was the 77A, between Aldwych and Wandsworth, which became the 87 in June 2006.