[biblio] Brackets inside brackets

[Pedro Costa]

Hi,

 In this example of a bibtex entry, the title fieldhas the value "{The
 {X} Window System, Version 11}". Why X is between { }? What does it
 means?

 @article{Gettys90,
   author = {Jim Gettys and Phil Karlton and Scott McGregor},
   title = {The {X} Window System, Version 11},
   journal = {Software Practice and Experience},
   volume = {20},
   number = {S2},
   year = {1990},
   abstract = {A technical overview of the X11 functionality.  This is an update
 of the X10 TOG paper by Scheifler \& Gettys.}
}

-- 
Thanks,