diff --git a/README.md b/README.md
index a4ce7e3..36bb217 100644
--- a/README.md
+++ b/README.md
@@ -28,5 +28,7 @@ Cite the following work thus:
 
 * Giuseppe G. A. Celano. (2017). Tokenized and sentence-splitted CTSized Ancient Greek texts (v1.1.0) [Data set]. Zenodo. http://doi.org/10.5281/zenodo.438311
 
+* Christopher Blackwell and Neel Smith, eds. (2014). The Canonical Text Services (CTS) Protocol Specification (v.5.0.rc.2). http://cite-architecture.github.io/cts_spec/
+
 # License
 <a rel="license" href="http://creativecommons.org/licenses/by-nc/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc/4.0/88x31.png" /></a><br />This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc/4.0/">Creative Commons Attribution-NonCommercial 4.0 International License</a>.