diff --git a/zip-0321.rst b/zip-0321.rst
index 874d8927..08539466 100644
--- a/zip-0321.rst
+++ b/zip-0321.rst
@@ -100,7 +100,9 @@ URI Syntax:
 
 Here, ``ALPHA``, ``unreserved`` and ``pct-encoded`` are as defined in
 [#RFC3986]_. "base58" is defined as in [#base58check]_. "base64url" is defined
-as in [#base64url]_ with padding omitted.
+as in [#base64url]_ with padding omitted. (Note that this uses a different
+alphabet to the usual base64; the values 62 and 63 in the alphabet are encoded
+as ``-`` and ``_`` respectively.)
    
 A ZIP-321 URI represents a request for the construction of a transaction having
 one or more *payments*. In the case that only a single payment is being