L C Descriptive Rules: 

Library of Congress. Rules for Descriptive cataloging at the Library of Congress Adopted by the American Library Association. - Washington, D.C.: Library of Congress, 1949. As evident from the title, it was adopted and given official recognition by the American Library Association as the standard for descriptive cataloguing. As a natural consequence, it replaced the second part (i.e., descriptive rules) in 1949 ALA rules, 2nd definitive edition which was also published by the ALA in the same year.