Much has been made of the importance of the tritone in the War Requiem. Not only does this interval colour many of the harmonies and melodic contours of the work, it is also an important agent in maintaining its characteristic tonal instability. Interestingly the composer's first large-scale masterpiece for voices and orchestra, Peter Grimes, is if anything even more dependent on tritone relationships, although unlike the Requiem it has few passages which are not either firmly in a key or else moving purposefully towards one. It could be said that whereas this interval provokes uncertainty in the later work, in Grimes it embodies incompatibility, for the opera's dramatic movement is generated by tonal progressions which swing back and forth between two great poles of A and E flat, keys at opposite ends of the tonal spectrum, to symbolise the impossibility of coexistence for Grimes and the Borough.