It's reserved for the show's biggest secrets, but the makers of Doctor Who have long used codenames and anagrams when they want to keep something from us. Most recently, Jenna-Louise Coleman revealed that she had to pretend she was auditioning for something called "Men on Waves".
She told Radio Times: "When she was auditioning, Karen Gillan had been given a codename - Panic Moon, which is an anagram of Companion - so I worked out that Men on Waves is an anagram of Woman Seven, because this is the seventh series. Weirdly, seven is my lucky number and this is my seventh job."
Both Karen Gillan and Jenna-Louise Coleman were cast by the current Doctor Who showrunner, Steven Moffat, but he didn't introduce the concept to the show. Back in 2005, the very biggest secret of them all was the show itself: everybody wanted to know what the revived Doctor Who would look like and the production team went so far as to relabel the very few videos that had to leave the office.
Instead of "Doctor Who", they were labelled with an anagram: "Torchwood".
Russell T Davies liked that anagram so much that he used it as the real title for his next science-fiction drama, which he had originally planned to call Excalibur.
Usually these anagrams and codenames are not meant to become public. But just occasionally, they are worked directly into Doctor Who and seen or heard on screen.
So in Gridlock, an early episode of the 2007 series, the Face of Boe tells the Doctor: "You Are Not Alone". You then had to pay attention for another eight episodes but if you did, you got a big clue that someone was not who or what they seemed. In the episode Utopia, the Doctor meets a professor whose surname is YANA, who turns out to be another Time Lord.
It transpires Yana is the Master and this time the anagrams and the codes are also a little nod to the original Doctor Who series, which regularly hid the names of the Master and Davros.
In the days before every conceivable scintilla of information about Doctor Who could be examined online, the place most fans got any details from was Radio Times magazine. Cast lists and credits would be studied and the producers knew this.
So in 1988's Remembrance of the Daleks, the production team knew full well that listing a character as being played by Terry Molloy would tip off fans that Davros, creator of the Daleks, was back. So in the cast list they provided to Radio Times for that week, they billed the actor as Roy Tromelly.
Sylvester McCoy was the Seventh Doctor in that story but the practice was in place with the Fifth and even the Fourth, too.
Some 29 years ago, Fifth Doctor (Peter Davison) story The King's Demons in 1983 co-starred James Stoker. It's an anagram of "Master's joke".
The year before saw it happen twice: both times with the Master and both times playing on the name of the actor, Anthony or Tony Ainley. In Time-Flight the Master was credited as being played by Leon Ny Taiy.
In Davison's very first story, Castrovalva, the newly regenerated Fifth Doctor believes he has found sanctuary and a friend in the form of the gentle, mumbling, wise old Portreeve, played by Neil Toynay.
No such codename or anagram trickery was needed for Tony Ainley's first appearance in the show as nobody even knew that the Master was going to appear in The Keeper of Traken. Ainley co-starred in this Fourth Doctor (Tom Baker) story as Tremas, the father of Nyssa. But in the last moments of the tale, Tremas was killed by the Master, who took over his body.
Tremas should've seen it coming: his name is an anagram of Master.
If the practice was used in the 1980s to fool viewers, in the 1970s it was more to fool - or at least pacify - the BBC. There was a rule preventing producers and script editors on the show, who were then BBC staff, commissioning scripts from themselves. You weren't allowed to hire yourself and that was quite fair.
But sometimes it was necessary to break the rule because of production problems or because scripts went wrong and had to be replaced at the last moment.
Most famously, the 1978 Tom Baker story, The Invasion of Time, was a last-minute script written to replace another serial that was going to cost too much to produce. It's famous because it's the story where a huge amount of time is spent walking around a Tardis that was revealed to have a lot of brick walls and look a great deal like the nearest location the show could find.
The Invasion of Time was written by script editor Anthony Read and producer Graham Williams but it was credited on screen and in Radio Times as being by David Agnew.
Williams again collaborated on a "David Agnew" script in 1979, this time City of Death with Douglas Adams.
There were several other occasions when the only solution to a script problem was for the production team to work together to write a new one. But there was also one time when it was done just a little less harmoniously.
Terrance Dicks so objected to rewrites of his 1976 story, The Brain of Morbius, that he insisted his name be taken off it. Script editor Robert Holmes asked what name he wanted used instead and Dicks reports that he just said to make it "a bland pseudonym" before hanging up the phone. To this day, Doctor Who episode guides credit The Brain of Morbius to writer Robin Bland.
Perhaps the very first example, certainly the first to have been revealed outside the programme, is actor Sydney Wilson. Again, there's no such person and today a swift check of the Radio Times film database would tell you that.
But back in 1965, when Doctor Who itself was only two years old, there was a story called The Rescue. Sorry to spoil it 47 years later, but don't trust Bennett. He may seem to be the good guy trying to protect Vicky (Maureen O'Brien) from the evil Koquillion, but he's got another agenda. He is Koquillion and that little surprise would've been wrecked by the cast list.
Consequently, for Desperate Measures, the second of this two-part story, Koquillion was credited as being played by Sydney Wilson. It's a combination of the names Sydney Newman and Donald Wilson. Newman was a BBC drama boss and Wilson was a producer who led the development of Doctor Who in its very earliest days.