Useful definition of terms, from John Carter's ABC For Book Collectors (8th Edition, 2004):

Provenance - The pedigree of a book's previous ownership. This may be clearly marked by the owner's name, arms, bookplate, or other evidence in the book itself; it may be less clearly indicated by press-marks; or it may have to be pieced together from such outside sources as auction records or booksellers' catalogues.

Association Copy - A copy which once belonged to, or was annotated by, the author; which once belonged to someone connected with the author or someone of interest in his own right; or belonged to someone peculiarly associated with its contents.

A-Grade: An A-grade signature is one that has the majority of the following:
  • very detailed provenance of the signed book or letter
  • is being sold by a knowledgeable and trusted vendor
  • letter is on Tolkien's personal stationery
  • has supporting materials or is part of a detailed letter that meets the norms for its type and has characteristics consistent with other period-correct items.

B-Grade: A B-grade signature is one that lacks detailed provenance, but has many of the following attributes:
  • an association copy - a letter from the collection of the recipient; a book from an author Tolkien knew and/or would have had in his library
  • stated authenticity from a trusted vendor or auction house
  • meets all the norms and characteristics expected

C-Grade: A C-grade signature has:
  • many of the correct norms and characteristics
  • lacks any sense of provenance or stated authenticity from a reliable source
  • may have one or two atypical characteristics that start to cause doubts

D-Grade: A D-grade signature will have many of the following
  • no provenance or association
  • is not being sold by a known dealer or auction house
  • has no stated authenticity from a reliable source
  • has many norms or characteristics that are atypical.

These grades are personal opinion and not to be relied upon, as is all commentary on signatures on the Tolkien Collectors Guide.

No representation or warranty, express or implied, is made nor responsibility of any kind is accepted by the Tolkien Collectors Guide and/or its members either as to the accuracy or completeness of any information stated.

Adapted and expanded on the grading system from this article on the Agatha Christie Collecting Site.