I think I mention these in one of my articles. 4 times a tear a representative from the diocese visited each parish within the diocese and made a transcript (copy) of the entries in the registers of baptism, marriage and burial. the parish registers were started by henryviii to piss off the pope but they where not very uniform nor very well maintained, Elizabeth I decided in 1598 that her church would have the best system in the known world and so she changed the type of media from parchment to velum and the way the books were indexed and lined and also how they were stored. then she decided that the local incumbents might not be uniform or reliable and so she ordered the quarterly visitations to copy the previous 3 months and these copies were kept at the bishops palace and subsequently tend to be the ones that survived the damp, rodents and theft. the BTs are the primary source the LDS used in the 50s and 60s to compile their igi.