A Pictish stone is a type of monumental stele, generally carved or incised with symbols or designs. A few have ogham inscriptions. Located in Scotland, mostly north of the Clyde-Forth line and on the Eastern side of the country, these stones are the most visible remaining evidence of the Picts and are thought to date from the 6th to 9th century, a period during which the Picts became Christianized. The earlier stones have no parallels from the rest of the British Isles, but the later forms are variations within a wider Insular tradition of monumental stones such as high crosses. About 350 objects classified as Pictish stones have survived, the earlier examples of which holding by far the greatest number of surviving examples of the mysterious symbols, which have long intrigued scholars. Classification In The Early Christian Monuments of Scotland (1903) J Romilly Allen and Joseph Anderson first classified Pictish stones into three groups. Critics have noted weaknesses in this system but it is widely known and still used in the field. In particular, the classification may be misleading for the many incomplete stones. Allen and Anderson regarded their classes as coming from distinct periods in sequence, but it is now clear that there was a considerable period when both Class I and II stones were being produced. Class I — unworked stones with symbols only incised. There is no cross on either side. Class I stones date back to the 6th, 7th and 8th century. Class II — stones of more or less rectangular shape with a large cross and symbol(s) on one or both sides. The symbols, as well as Christian motifs, are carved in relief and the cross with its surroundings is filled with designs. Class II stones date from the 8th and 9th century. Class III — these stones feature no idiomatic Pictish symbols. The stones can be cross-slabs, recumbent gravemarkers, free-standing crosses, and composite stone shrines. They originate in the 8th or 9th century. Historic Scotland describes this class as "too simplistic" and says "Nowadays this is not considered a useful category. A surviving fragment may belong to a monument that did include Christian imagery". Later Scottish stones merge into wider medieval British and European traditions. Purpose and meaning The purpose and meaning of the stones are only slightly understood, and the various theories proposed for the early Class I symbol stones, those that are