In the current BBC Wales system, a production block is a device used by the production manager — in consultation with the producers, executive producers and senior department heads — to divide the principal photography schedule to maximise the efforts of behind-the-scenes personnel and actors. Instead of filming and contracting for single episodes, directors, script editors, directors of photography and some actors — along with many other individuals —are assigned to groups of episodes, which may later be broadcast in a completely different order. Reasons for combining certain episodes into production blocks include: availability of talent, the need to use the same locations in multiple episodes, the necessity of certain episodes to have the same sort of background weather, or a perceived ability to save money by filming certain episodes together. Production blocks always vary by the needs of a particular series and can overlap, as happens intentionally on the BBC Wales version of Doctor Who through double banking.
Production blocks are a staple feature of the BBC Wales version of Doctor Who and her sister shows. An obvious sign of the use of production block is the fact that at least a few episodes in each series have been broadcast in a different order than their production order. Generally, a production block consists of two episodes, but they have been as few as one as in the case of "Doctor-lite" and most "special" episodes or as many as four, as with Rise of the Cybermen/The Age of Steel/Army of Ghosts/Doomsday). Production blocks are assigned simple ordinal numbers, allowing clappers to identify filmed scenes not by name, but a numerical code which helps keep the identity of various filmed scenes from the public. The BBC Wales calendar sometimes — but not always — allows for production to take a short break between production blocks.
It should not be assumed that production blocks are an innovation of BBC Wales and thus "modern" television production. Doctor Who has always been divided into production blocks. Although the first serial to be broadcast out of order was The Curse of Peladon — thus proving the notion of a difference between the production and broadcast calendars — production blocks were used by the BBC at Television Centre and its predecessors from 1963. Its precise meaning evolved over time.
In the 1960s, a "production block" was, roughly, an entire season's worth of episodes. However, because seasons were much longer in the 1960s, one serial from the outgoing production block was held over to start the new broadcast season. This allowed the production team a "cushion" against unforeseen disasters. For example, Galaxy 4 was technically a part of the second production block, but broadcast as the opener to season 3. In early Doctor Who history, the equivalent to the BBC Wales "production block" was called a "recording block". This was generally around 6-8 episodes in length and tended to be staffed by the same people. A few good examples of a recording block are The Rescue/The Romans and The Time Meddler/Galaxy 4/Mission to the Unknown. Recording blocks were the basic division of the schedule in which most production personnel operated. Designers were often assigned to entire blocks, as with Raymond Cusick for The Rescue and The Romans. Actors' contracts were often conceived in terms of full or half recording blocks. These blocks or half blocks usually corresponded to whole serials — but not always. Due to restructuring of stories after contracts were signed, for instance, Jackie Lane, Michael Craze and Anneke Wills were all written out mid-serial.
At the instigation of Patrick Troughton and supported by Barry Letts, the way principal photography occurred was changed by season 7. At the same time, the number of episodes per season dropped dramatically beginning with the Jon Pertwee era. This new paradigm and technological advancement also gave the production team greater ability to film on location. The notion of "blocks" began to shift. A "recording block" came to be seen as "production block" and the words were used interchangeably.
Now a recording block was considered to be basically a season in length. Occasionally, though, the last episode of a recording/production block was held over for broadcast in the next season, as with The Time Warrior, Robot and Terror of the Zygons. Subdivisions of a production block were now thought of in terms of a serial's "studio blocks" and "location blocks". Thus, all the location work for a serial would be handled in one block and all the studio would be done in another. Beginning in the 1970s, principal actors' contracts were handed out in terms of what Verity Lambert would have considered "production blocks". The only exception to this in the 1970s came about because Elizabeth Sladen declined to negotiate in those terms for her final contract of the decade. Behind-the-scenes personnel began, to be assigned strictly to serials, rather than to particular blocks in the 1970s.
This practice for behind-the-scenes personnel continued through the 1980s. However, John Nathan-Turner had a different philosophy for actors than his 1970s counterparts. He revived the notion of sub-production block contracting in the 1980s. Almost every major character either debuted or exited in the midst of an ongoing production block. Aside from Tom Baker, the exceptions were generally those over whose departure Nathan-Turner had no control: Colin Baker, Sophie Aldred and Sylvester McCoy.
Though Doctor Who had begun as a series of recording blocks which did not necessarily correspond to single stories, by the 1970s the serial — divided into its studio and location blocks — had emerged as the dominant discrete unit of production scheduling. In many ways, a serial's production would be equivalent to the BBC Wales notion of a "production block". Nevertheless, the BBC Wales notion of a production block also has a lot in common with those earlier recording blocks, as seen in the William Hartnell era.