diff --git a/gpgmymail b/gpgmymail
index 03132a1..3bf1705 100755
--- a/gpgmymail
+++ b/gpgmymail
@@ -83,6 +83,19 @@ def decode_email(message: email.message.Message) -> email.message.Message:
         decoded_bytes: bytes,
         most_recent_boundary: str = None
     ) -> bytes:
+        """
+        change decoded_bytes such that part is decoded if base64 (unchanged if
+        not)
+
+        see usage below for examples
+
+        :param part: email.message.Message to be decoded
+        :param decoded_bytes: the email as a bytes object (ie not a string with
+                              encoding), will have modified version returned
+        :param most_recent_boundary: str of the most recent boundary so we
+                                     don't overwrite this
+        :return: bytes of decoded_bytes with part decoded if base64
+        """
         if part.get("Content-Transfer-Encoding") == "base64":
             b64_str = part.get_payload()
             # remove the boundary as we don't want to change this