diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 61f73e9..e83a4ac 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -1125,7 +1125,9 @@
 
         Creates a new SubjectKeyIdentifier instance using the public key
         provided to generate the appropriate digest. This should be the public
-        key that is in the certificate.
+        key that is in the certificate. The generated digest is the SHA1 hash
+        of the ``subjectPublicKey`` ASN.1 bit string. This is the first
+        recommendation in :rfc:`5280` section 4.2.1.2.
 
         :param public_key: One of
             :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey`