In the past the Library of Congress has attempted to ensure the uniqueness of each LC call number it assigns. With the implementation of the LC ILS on August 16, 1999, the Library will continue to ensure the uniqueness of call numbers it assigns in either field 050 or 051 in active bibliographic records. The mechanism used in the LC card shelflist that allowed the Library to ensure unique call numbers over time was to retain a card for each call number that was no longer actively in use for whatever reason (e.g., a work was reclassified; an item was withdrawn from the collections). There is no direct counterpart to this mechanism in the LC ILS. The Library considered alternatives but judged the resources required to ensure the uniqueness of call numbers currently assigned in relation to all call numbers ever assigned were disproportionate to the perceived benefit. Therefore, the Library can no longer guarantee that a call number once used (e.g., for a work now reclassified; in a bibliographic record that has now been canceled) will not be used again.