diff --git a/Doc/library/binascii.rst b/Doc/library/binascii.rst
index 5eb7c4b..38f616a 100644
--- a/Doc/library/binascii.rst
+++ b/Doc/library/binascii.rst
@@ -101,8 +101,10 @@
 
 .. function:: crc_hqx(data, crc)
 
-   Compute the binhex4 crc value of *data*, starting with an initial *crc* and
-   returning the result.
+   Compute a 16-bit CRC value of *data*, starting with an initial *crc* and
+   returning the result.  This uses the CRC-CCITT polynomial
+   *x*:sup:`16` + *x*:sup:`12` + *x*:sup:`5` + 1, often represented as
+   0x1021.  This CRC is used in the binhex4 format.
 
 
 .. function:: crc32(data[, crc])