seal. In the United States, the word "seal" is sometimes assigned to a facsimile of the seal design (in monochrome or color), which may be used in a variety of contexts including architectural settings, on flags, or on official letterheads. Thus, for example, the Great Seal of the United States, among other uses, appears on the reverse of the one-dollar bill; and several of the seals of the U.S. states appear on their respective state flags. In Europe, although coats of arms and heraldic badges may well feature in such contexts as well as on seals, the seal design in its entirety rarely appears as a graphical emblem and is used mainly as originally intended: as an impression on documents. The study of seals is known as sigillography or sphragistics. Ancient Near East Seals were used in the earliest civilizations and are of considerable importance in archaeology and art history. In ancient Mesopotamia carved or engraved cylinder seals in stone or other materials were used. These could be rolled along to create an impression on clay (which could be repeated indefinitely), and used as labels on consignments of trade goods, or for other purposes. They are normally hollow and it is presumed that they were worn on a string or chain round the neck. Many have only images, often very finely carved, with no writing, while others have both. From ancient Egypt seals in the form of , including some with the names of kings, have been found; these tend to show only names in hieroglyphics. Recently, seals have come to light in South Arabia datable to the Himyarite age. One example shows a name written in Aramaic (Yitsḥaq bar Ḥanina) engraved in reverse so as to read correctly in the impression. Ancient Greece and Rome From the beginning of the 3rd millennium BC until the Middle Ages, seals of various kinds were in production in the Aegean islands and mainland Greece. In the Early Minoan age these were formed of soft stone and ivory and show particular characteristic forms. By the Middle Minoan age a new set for seal forms, motifs and materials appear. Hard stone requires new rotary carving techniques. The Late Bronze Age is the time par excellence of the lens-shaped seal and the seal ring, which continued into the Archaic, Classical and Hellenistic periods, in the form of pictorial engraved gems. These were a major luxury