7. Glass
Let's go back to basics with parallel worlds. They're one of the most common ideas of sci-fi, and they were most famously used in NewWho when the Cybus Cybermen tore down the void to convert mankind and break up the Doctor and Rose. But whenever a writer or director wants to show us that there are two worlds - parallel or not - within the same story, they usually do so with glass or a mirror. The Doctor surveyed Pete's World in 'Doomsday' through a Canary Wharf window; likewise, he, Mickey and Rose watched Madame Pompadour through her mirror. More recently, there's glass between the two ships in 'The Curse of the Black Spot' and magnifying glasses signifying different timestreams in 'The Girl Who Waited'. The Doctor's even learned the importance of windows now, as in 'The God Complex', it's the second thing he looks for after a door, although the spinning mirror thing in 'Hide' didn't really advance the plot of the pocket universe all that much. And when the walls between universes are compromised? The Doctor cracks the Torchwood office glass with his sonic screwdriver to further link the imagery, and famously smashes Reinette's mirror with Arthur, the horse (spin-off for Arthur and Susan/Joshua pleeease?). Even the TARDIS windows in 'The Name of the Doctor' are cracked, indicating that another world - the past - can now be accessed. It's all subtle stuff, but the fragility of glass is a lovely metaphor for how delicately the Doctor has to tread when switching between linked settings. Rose, if you want to keep TenTwo all to yourself, get yourself the sturdiest double glazing.