pxcor pycor
pxcor pycor 
These are built-in patch variables. They hold the x and y coordinate of the patch. They are always integers. You cannot set these variables, because patches don't move.
pxcor is greater than or equal to min-pxcor and less than or equal to max-pxcor; similarly for pycor and min-pycor and max-pycor.
All patch variables can be directly accessed by any turtle standing on the patch.