What  is  the difference between the subject  headings  "Judaism--20th  century" and  "Judaism--History--20th century."
 
Both headings are valid and  one  will  find  both applied to  LC  records.
 
"Judaism--20th century" was established for  works which describe aspects of contemporary   Judaism.    "Judaism--History--20th  century,"  a  valid application of  the free-floating string,  is  assigned to  works which present an historical treatment of  20th century Judaism.

LC  is currently discussing  the use of the subdivision "--History" as  an outgrowth of Airlie House Conference proposals regarding subdivision simplification. In  the course of  the discussions, some have questioned why,  in general, free-floating century subdivisions could not be assigned  without interposing "--History--."  Often  we  are forced to  use  "History" even though a work is  not  an historical treatment per  se, but simply describes the topic in a particular period.
 
After topical headings in  certain disciplines and  after certain  subdivisions, period subdivisions (established  or free-floating)
are assigned without interposing "History."  While  this  is  not  generally  the  case  with  individual  religions, "Judaism--20th
century remains a valid (vestigial) heading.
                                                                    
Lenore  Bell