This is an extract from the bash manual obtained by typing 'man bash'.

BASH(1) 							       BASH(1)

   Appending Redirected Output
       Redirection  of	output	in  this  fashion  causes  the file whose name
       results from the expansion of word to be opened for appending  on  file
       descriptor  n,  or  the standard output (file descriptor 1) if n is not
       specified.  If the file does not exist it is created.

       The general format for appending output is:

	      [n]>>word