1 files changed, 4 insertions, 0 deletions
diff --git a/README.md b/README.md
index 5ed4136..4f8d8ea 100644
--- a/README.md
+++ b/README.md
@@ -45,6 +45,10 @@ Entries in this repository also uses the following non-standard BibTeX fields:
 
 * `oclc`: OCLC Control Number - https://en.wikipedia.org/wiki/OCLC.
 
+* `ucd`: Universal Decimal Classification - https://en.wikipedia.org/wiki/Universal_Decimal_Classification.
+
+* `ddc`: Dewey Decimal Classification - https://en.wikipedia.org/wiki/Dewey_Decimal_Classification.
+
 ## Testing
 
     sudo apt install make biber pybtex