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