You can use the Snap features to align your brush strokes and objects precisely to the nearest grid line or guide. The snap influence controls how many pixels away an item must be to snap to guide or grid.
Note: An item’s center point will snap to a guide or grid point if the center point is the closest part of the item to a grid point or guide. Grids or guides must be displayed for items to snap to them.
To snap to the nearest grid line:
Choose View > Snap to Grid (Ctrl + Shift + G).
To snap to the nearest guide:
Choose View > Snap to Guides (Shift + Alt + G).
Note: If a grid point and a guide are equal distances from an item, the item will snap to the guide.
To change the snap influence:
Choose View > Change Grid, Guide, and Snap Properties.
Click the Grid or Guides tab
and change the Snap influence value.
The Default settings group box shows
settings for all future guides or grids. The Current
image settings group box controls settings for guides in the current
image.
Click OK.