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The Tain of China Miéville's title refers to a thin layer of silver or tin foil such as is used on the backing of a mirror.
In this case, the tain in question divides the world of humans from the world of imagos, which are never entirely defined by Miéville, but which are clearly antithetical to humans and live in a sort of mirror world...usually.
Some time prior to the beginning of the story, the imagos have managed to breach the tain between their world and ours, laying waste to vast tracts of London, which is now sparsely populated by imagos and humans alike, although in separate regions.
Tain (Modern Gaelic: Baile Dhubhthaich, Duthac's town) is a royal burgh in the committee area of Ross and Cromarty, in the Highland area of Scotland.
Tain was granted its first royal charter in 1066, making TainScotland's oldest Royal Burgh, an event commemorated in 1966 with the opening of the Rose Garden by Her Majesty Queen Elizabeth, the Queen Mother.