diff --git a/README.adoc b/README.adoc
index b79d4ba..eb777a5 100644
--- a/README.adoc
+++ b/README.adoc
@@ -143,8 +143,9 @@ Client Certificates
 certificate specified by the respective server's `tls_cert` option if you add
 `sasl` to the `capabilities` option and the server supports this.
 
-'xD' uses SHA-256 fingerprints of TLS client certificates to authenticate users.
-To get the fingerprint from a certificate file in the required form, use:
+'xD' and 'xS' use SHA-256 fingerprints of TLS client certificates
+to authenticate users.  To get the fingerprint from a certificate file
+in the required form, use:
 
  $ openssl x509 -in public.pem -outform DER | sha256sum