what is the black grid over the gray grid ?

[Cristian SecarÄ <orice secarica ro>]

Apart from the light-gray grid, which can be turned on and off, there
is another sparse black grid over it that I cannot get rid of.
What is its purpose ? Its dimension is not even a multiple of
something, it falls in random places with reference to rulers.

??

Thank you,
Cristi

-- 
Cristian SecarÄ
http://www.secarica.ro/