There are occasions when a writer's meaning may be conveyed more exactly by (say) not unkindly, not unnaturally or not unjustifiably, than by kindly, naturally or justifiably. But the "not un-" habit is liable to take charge, with disastrous effects, making the victim forget all straightforward adjectives and adverbs. George Orwell recommended that we should all inoculate ourselves against the disease by memorising this sentence: " A not unblack dog was chasing a not unsmall rabbit across a not ungreen field ". See also DE and DIS and NON for the practice of creating new words by what are called "privative prefixes".