Explain that FILETYPE_ASN1 is DER.
diff --git a/doc/api/crypto.rst b/doc/api/crypto.rst
index 41b659b..0568ad7 100644
--- a/doc/api/crypto.rst
+++ b/doc/api/crypto.rst
@@ -35,7 +35,12 @@
---------------------------------
The following serialization functions take one of these constants to
-determine the format:
+determine the format.
+
+:data:`FILETYPE_PEM` serializes data to a Base64-encoded encoded representation
+of the underlying ASN.1 data structure. :data:`FILETYPE_ASN1` serializes data
+to the underlying ASN.1 data structure. The format used by
+:data:`FILETYPE_ASN1` is also sometimes referred to as DER.
.. py:data:: FILETYPE_PEM
FILETYPE_ASN1