Takes an integer number or 'g' or 'c' as its argument. A number is taken as the number of the Xinerama screen that is to be used as the primary screen. The primary screen can be used as the preferred screen to place windows with

XineramaPrimaryScreen <screen number>
Style * StartsOnScreen p

The primary screen is used in some of the modules and for the default icon box too. Any number that is zero or more is taken as the primary screen's number. Instead, the letter 'c' indicates to use the current screen (containing the pointer) whenever the primary screen is used. This may be very confusing under some circumstances. With 'g', the global screen is used as the primary screen, effectively disabling the primary screen. Calling this function with any other argument (including none) resets the primary screen to 0.