The Chicago Assyrian Dictionary was conceived to provide more than lexical information alone, more than a one-to-one equivalent between Akkadian and English words. By presenting each word in a meaningful context, usually with a full and idiomatic translation, it recreates the cultural milieu and thus, in many ways, assumes the function of an encyclopaedia. Its source material ranges in time from the 3rd millennium BC to the 1st century AD and in geographic area from the Mediterranean Sea in the west to the Zagros Mountains in the east. The Chicago Assyrian Dictionary has become an invaluable source for the study of the civilisations of the ancient Near East; their political and cultural history; their achievements in the sciences of medicine, astronomy, mathematics and linguistics; and not least, the timeless beauty of their poetry. Volume 21, alphabetically the last, was published in the early years of the project; Volume 20: U/W is the final volume and its publication marks the completion of the set.