The first edition of Roget's Thesaurus of English Words and Phrases was published in 1852 and is still one of the world's most popular reference books. Peter Mark Roget's original structure has changed little in subsequent editions, even to accommodate the many scientific and technological changed which have taken place. His classification system is the unique feature of the Thesaurus and took him nearly 50 years to perfect. Roget's Thesaurus was not published until he was 73 years old. Only 1000 copies of the first edition were printed. This facsimile edition, bound in cloth also contains an introduction by Laurence Urdang and gives an insight into Roget's achievement.