INTRODUCTION 



A book can stand in only one place on the shelves and 

 can have but one class number. Its title, however, may be 

 repeated in a catalogue in as many classes as it has subjects. 

 When entered in the class of its main subject, the number is- 

 printed in ordinary roman type ; when entered also in any 

 other class (where the book does not stand) the number is- 

 printed in italics. Thus : 



616 PATHOLOGY, 



'616 : 6 1 1 Pathological Anatomy. 



611 ANATOMY, 



'616 : 611 Pathological Anatomy. 



In addition to the significant class number, each book 

 receives a book mark which serves to distinguish it from all 

 other books belonging to the same class. This book mark 

 almost always consists of the first three letters of the 

 author's surname, or of a word selected, according to certain 

 rules, from the title. As the author's surname or the selected 

 word appears first in the clarendon type at the beginning of 

 each entry of the catalogue, the book mark has not been 

 reprinted at the side. In the case, however, of periodicals 

 and publications of societies, where the same first words 

 (Journal, Annals, etc.) recur with frequency, it is necessary 

 to choose, more or less arbitrarily, certain letters, usually 

 initials, to represent the titles. These letters are printed 

 immediately after the class number of each book. Thus : 



504 Helmholtz, A. von. Popular lectures on scien- 



tific subjects. 



5(26) Challenger. Report on scientific results of 

 voyage. 



5805 A.B. Annals of botany. 



5806 L.J. London. Linnean Society. Journal. 



The actual book mark of the first of these is Hel, of the 

 second Cha. 



