diff --git a/src/types.ml b/src/types.ml
index 4eb0d672..f4546537 100644
--- a/src/types.ml
+++ b/src/types.ml
@@ -131,7 +131,9 @@ module Eddsa = struct
     pub |> Mirage_crypto_ec.Ed25519.pub_to_octets |> Base_32.encode
 
   (* EdDSA signatures are transmitted as 64-bytes base32
-   binary-encoded objects with just the R and S values (base32_ binary-only). *)
+     binary-encoded objects with just the R and S values (base32_ binary-only).
+
+     They are signature over a c-struct like `TALER_xxxPS` + with a purpose *)
   type signature = string
 end