How do I model across a mirror modifier? I am modeling on a set of windows right now, and they are not symmetrical along the X axis, where I need the mirror modifier to mirror the other half of the superstructure. This is what it looks like. The problem basically arises when I need to make the windows non-symmetrical across the mirror. 

One possible way would probably be to apply the mirror, make the adjustments, then separate the windows from the main superstructure and finally joining them with the superstructure afterwards?
Solution: Separate the geometry that will be non-symmetrical, and rename it.


Select the object (by right-clicking on it), then press Tab on the keyboard to enter Edit mode. 
Then, select the geometry that you want to separate, and press P (hotkey for separating) and you are done! 


Now you can press Tab again to go back into Object mode and see the separated geometry.  

You can also rename a mesh by selecting it in the Outliner edtior (the object list in the far up-right in the default window layout), double-click the object's name and type in the new name instead of the one with a ".001" at the end given to the object by default.