-EVP_PKEY_type() returns the type of key corresponding to the value
-B<type>. The type of a key can be obtained with
-EVP_PKEY_type(pkey->type). The return value will be EVP_PKEY_RSA,
-EVP_PKEY_DSA, EVP_PKEY_DH or EVP_PKEY_EC for the corresponding
-key types or NID_undef if the key type is unassigned.
+EVP_PKEY_base_id() returns the type of B<pkey>. For example
+an RSA key will return B<EVP_PKEY_RSA>.
+
+EVP_PKEY_id() returns the actual OID associated with B<pkey>. Historically keys
+using the same algorithm could use different OIDs. For example an RSA key could
+use the OIDs corresponding to the NIDs B<NID_rsaEncryption> (equivalent to
+B<EVP_PKEY_RSA>) or B<NID_rsa> (equivalent to B<EVP_PKEY_RSA2>). The use of
+alternative non-standard OIDs is now rare so B<EVP_PKEY_RSA2> et al are not
+often seen in practice.
+
+EVP_PKEY_type() returns the underlying type of the NID B<type>. For example
+EVP_PKEY_type(EVP_PKEY_RSA2) will return B<EVP_PKEY_RSA>.