Snap, or Aero snap is a feature which is introduced in Windows 7, which allows you to quickly move or resize open documents and arrange it according to your needs.
Using the snapping feature you can drag windows to the sides of the screen using a mouse such that each takes half the screen, allowing you to work simultaneously on both the windows in parallel.
On single windows it is quite simple, as dragging the window by its top bar to the top two corners expand it to cover half of the screen each. However, if you are using your windows 7 using multiple screens, you might face some problems as the mouse is rendered useless. In such a scenario, you can use the keyboard shortcuts for this to the maximum effect. These shortcuts work for a single screen too.
If you have a window opened up, press Start + Up direction key to maximize it on the screen.
Or alternately, press Start + Left direction key to snap the window to the left half of the display.
Now, if you press Start + Right direction key, the window reverts to its original state, while pressing the same combination of keys again snaps the window to the right half of the display.
If the page is maximised, then Start + Down key reverts it to its original state, and another press of the same combination keys brings it to a minimised state. Then you will have to use the mouse to open the window again.
Snap can be used against multiple screens as well. In such scenarios the keyboard is used as while each display may have the mouse at only 50% of the time, the keyboard is accessible to both the display screens.
