--- a/doc/interpreter/arith.txi
+++ b/doc/interpreter/arith.txi
@@ -118,6 +118,20 @@
 example, @code{sin} expects an angle in radians while @code{sind} expects an
 angle in degrees.
 
+Octave uses the C library trigonometric functions.  It is expected that these
+functions are defined by the ISO/IEC 9899 standard. This Standard is available
+at: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf.  Section F.9.1 deals
+with the trigonometric functions.  The behavior of most of the functions is
+relatively straightforward. However, there are some exceptions to the standard
+behavior.  Many of the exceptions involve the behavior for -0.
+The most complex case is atan2. Octave exactly implements the behavior given
+in the Standard.  Including @tex $atan2(\pm0, -0)$ @end tex @ifnottex @code 
+{atan2(+- 0, 0)} @end ifnottex
+returns @tex $\pm \pi.$@end tex @ifnottex @code{+- pi}.
+
+It should be noted that Matlab uses different definitions which apparently
+do not distinguish -0.
+
 @DOCSTRING(sin)
 @DOCSTRING(cos)
 @DOCSTRING(tan)