(1964) indicates that traction is provided by a large, omnidirectional rotating metal sphere, while in the serial Death to the Daleks (1974) the Doctor states that they move by psycho-kinetic power. For years Daleks were depicted as being unable to climb stairs, with travel confined to smooth ground and level surfaces. This was rectified in Revelation of the Daleks in which a hovering Dalek was seen briefly, and then in Remembrance of the Daleks which featured a Dalek levitating up a staircase. From 2005 various Doctor Who episodes have shown Daleks hovering and flying with ease, locomotion being imparted by anti-gravity generators. Dalek variant naming conventions During the course of the Doctor Who television programme the BBC, its producers and scriptwriters have rarely alluded to changes in Dalek design or ascribed names or designations to the various models seen. Notable exceptions usually refer to rank, such as "Emperor" and "Supreme Dalek", although in Genesis of the Daleks the newly created Dalek casing is referred to as a "Mark 3 Travel Machine". The Dalek variant naming conventions used in this article are attributed to the various models as a matter of convenience. They can be found in general use by the Doctor Who/Dalek fan base, and are often quoted in posts and documentation on websites such as The Project Dalek Forum. Other classifications for the major Dalek variants exist. The Doctor Who Technical Manual (1983) places both television and Movie Daleks in the same numerical sequence by order of their first screen appearances, although reconciling the characters and events depicted in the Doctor Who films with those established in the TV programme is problematic. The term "drone" is used in this article to indicate a Dalek having no rank. Its only use to date in the Doctor Who television serials and films in which the Daleks have appeared has been in the episodes "Victory of the Daleks" and "Revolution of the Daleks". Mark 1 Daleks The Daleks first appeared in the 1963 Doctor Who serial The Daleks. There are no visual cues to distinguish one Dalek from another, or suggestions in the story of a hierarchy. The Mark 1 Dalek differs from later variants in having no shoulder slats or mesh, nine eye discs (the greatest number of any version) and a "toffee apple" shaped eyeball. The eye lenses are illuminated and shown to have an iris which can expand and