(i) B L V S N (ii) H D T C QU (iii) M G NG F R (iv) A O U E I ] The above diagram shows the Ogam alphabet arranged in four groups. A few other characters are occasionally used. It will be noted that crude as this device for writing is, it is applied with considerable skill and is framed with much ingenuity. The simpler sounds are represented by simpler letters than the more complex. Letters in frequent use like the vowels are the easiest to form. The arrangement of the vowels is different from that used in the Latin and English alphabets but corresponds to the more scientific arrangement adopted by modern phoneticians. It is worth noting that the characters in group (ii) stand for the initial sounds of the Old Irish words for _one_, _two_, _three_, _four_, _five_, in the order given. The question of Ogam writing has occupied the attention of many antiquarians and though some of the theories projected by the earlier investigators have been rejected as untenable our knowledge of Ogam at the present time is fairly definite but somewhat limited. It was once thought that Ogam was a cryptic alphabet, but many of the inscriptions have been read by means of the above key which is to be found in the Book of Ballymote. Owing to the fact that a few of the later Ogams have been found with duplicates in Roman letters they have been deciphered independently. As to the distribution of these Ogam inscribed stones it is worthy of note that in Ireland they are found chiefly in the south-west, and in Britain they are confined to those parts where it is known the Irish Gael had settled.[22] Owing to the lack of criteria for dating certain sound-changes in the Irish language it is impossible in the present stage of our knowledge to assign definite chronological limits to these Ogam inscriptions.[23] Mr. Quiggin in his account of Ogam[24] asserts that the earlier inscriptions cannot be later than the fifth century and if pagan they may be a century or two earlier. All Ogam inscriptions with accompanying Roman letters he would assign to a later period than 500 A.D. with the sole exception of the bilingual inscription of Killen Cormac (Cillin Cormac) which is believed to be earlier than 500 A.D.[25] Over three hundred Ogam inscriptions have been found and where they have not been injured or defaced they can generally be interpreted. Heroes and druids in the older epics are represented as making constant use of Ogam letters, sometimes inscribing them on wooden staves. The state of civilization depicted in these ancient poems and prose narratives seems to belong mainly to pre-Christian Ireland.