====== Grid and Guides ======
refer: 
http://helpx.adobe.com/en/photoshop/using/grid-guides.html
===== Show or hide a grid, guides, or smart guides =====
Do one of the following:
  * Choose View > Show > Grid.
  * Choose View > Show > Guides.
  * View > Show > Smart Guides.
  * Choose View > Extras. This command also shows or hides layer edges, selection edges, target paths, and slices.
===== Place a guide =====
  - If the rulers are not visible, choose View > Rulers. //**Note**:For the most accurate readings, view the image at 100% magnification or use the Info panel.//
  - Do one of the following to create a guide:
      * Choose View > New Guide. In the dialog box, select Horizontal or Vertical orientation, enter a position, and click OK.
      * Drag from the horizontal ruler to create a horizontal guide.
      * Drag from the vertical ruler to create a vertical guide.
  - (Optional) If you want to lock all guides, choose View > Lock Guides.

===== Move a guide =====
  - Select the Move tool , or hold down Ctrl (Windows) or Command (Mac OS) to activate the Move tool.
  - Position the pointer over the guide (the pointer turns into a double-headed arrow).
  - Move the guide in any of the following ways:
      * Drag the guide to move it.
      * Change the guide from horizontal to vertical, or vice versa, by holding down Alt (Windows) or Option (Mac OS) as you click or drag the guide.
      * Align the guide with the ruler ticks by holding down Shift as you drag the guide. The guide snaps to the grid if the grid is visible and View > Snap To > Grid is selected.
===== Remove guides from the image =====
Do one of the following:
  * To remove a single guide, drag the guide outside the image window.
  * To remove all guides, choose View > Clear Guides.
===== Set guide and grid preferences =====
  - Do one of the following:
    * (Windows) Choose Edit > Preferences > Guides, Grid, & Slices.
    * (Mac OS) Choose Photoshop > Preferences > Guides, Grid, & Slices.
  - For Color, choose a color for the guides, the grid, or both. If you choose Custom, click the color box, choose a color, and click OK.
  - For Style, choose a display option for guides or the grid, or both.
  - For Gridline Every, enter a value for the grid spacing. 
    * For Subdivisions, enter a value by which to subdivide the grid.  
    * If desired, change the units for this option. The Percent option creates a grid that divides the image into even sections. For example, choosing 25 for the Percent option creates an evenly divided 4‑by‑4 grid.
  - Click OK.