diff --git a/src/include/mb/pg_wchar.h b/src/include/mb/pg_wchar.h
index d456309..1148eb5 100644
--- a/src/include/mb/pg_wchar.h
+++ b/src/include/mb/pg_wchar.h
@@ -37,6 +37,31 @@ typedef unsigned int pg_wchar;
 #define ISSJISTAIL(c) (((c) >= 0x40 && (c) <= 0x7e) || ((c) >= 0x80 && (c) <= 0xfc))

 /*
+ * Currently PostgreSQL supports 5 types of mule internal encodings:
+ *
+ * 1) 1-byte ASCII characters, each byte is below 0x7f.
+ *
+ * 2) "Official" single byte charsets such as ISO 8859 latin1.  Each
+ *    mule character consists of 2 bytes: LC1 + C1, where LC1 is
+ *    corresponds to each charset and in range of 0x81 to 0x8d and C1
+ *    is in rage of 0xa0 to 0xff(ISO 8859-1 for example, plus each
+ *    high bit is on).
+ *
+ * 3) "Private" single byte charsets such as SISHENG.  Each mule
+ *    character consists of 3 bytes: LCPRV1 + LC12 + C1 where LCPRV1
+ *    is either 0x9a (if LC12 is in range of 0xa0 to 0xdf) or 0x9b (if
+ *    LC12 is in range of 0xe0 to 0xef).
+ *
+ * 4) "Official" multibyte charsets such as JIS X0208.  Each mule
+ *    character consists of 3 bytes: LC2 + C1 + C2 where LC2 is
+ *    corresponds to each charset and is in rage of 0x90 to 0x99. C1
+ *    and C2 is in rage of 0xa0 to 0xff(each high bit is on).
+ *
+ * 5) "Private" multibyte charsets such as CNS 11643-1992 Plane 3.
+ *    Each mule character consists of 4 bytes: LCPRV2 + LC22 + C1 +
+ *    C2.  where LCPRV2 is either 0x9c (if LC12 is in range of 0xf0 to
+ *    0xf4) or 0x9d (if LC22 is in range of 0xf5 to 0xfe).
+ *
  * Leading byte types or leading prefix byte for MULE internal code.
  * See http://www.xemacs.org for more details.	(there is a doc titled
  * "XEmacs Internals Manual", "MULE Character Sets and Encodings"