Production block In the current BBC Wales system , a production block is a device used by the production manager — in consultation with the producer s , executive producer s 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 , director s , script editor s , 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 . History . 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 clapper s 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 .