1. A method of decoding a bitstream of encoded data to reconstruct a significance map for a transform unit, the method comprising: for each bit position in the significance map having a significant coefficient flag that is to be decoded using context-adaptive decoding, determining a context for that bit position based upon a partition set, decoding the encoded data based on the determined context to reconstruct a bit value, and updating the context based on that reconstructed bit value, wherein the reconstructed bit values form the decoded significance map; and wherein the partition set assigns contexts to bit positions within the significance map such that each bit position in an upper left quadrant of the significance map is assigned one of at least four contexts, an upper right and lower left quadrant of the significance map are assigned four to six contexts different from said at least four contexts, wherein each of the four to six contexts is assigned to at least two bit positions, and a majority of bit positions in a lower right quadrant are assigned a single context.