--- a/doc/interpreter/signal.txi	Mon Jan 12 10:42:03 2009 +0100
+++ b/doc/interpreter/signal.txi	Mon Jan 12 10:54:45 2009 +0100
@@ -23,6 +23,18 @@
 
 @DOCSTRING(fft)
 
+Octave uses the FFTW libraries to perform FFT computations. When Octave
+starts up and initializes the FFTW libraries, they read a system wide
+file (on a Unix system, it is typically @file{/etc/fftw/wisdom}) that
+contains information useful to speed up FFT computations.  This
+information is called the @emph{wisdom}.  The system-wide file allows
+wisdom to be shared between all applications using the FFTW libraries.
+
+Use the @code{fftw} function to generate and save wisdom.  Using the
+utilities provided together with the FFTW libraries
+(@command{fftw-wisdom} on Unix systems), you can even add wisdom
+generated by Octave to the system-wide wisdom file.
+
 @DOCSTRING(fftw)
 
 @DOCSTRING(ifft)