Use pixel snapping

To turn on pixel snapping, use the Snap To Pixels command in the View menu. If Snap To Pixels is on, a pixel grid appears when the view magnification is set to 400% or higher. The pixel grid represents the individual pixels that appears in your Flash application. When you create or move an object, it is constrained to the pixel grid.

If you create a shape whose edges fall between pixel boundaries—for example, if you use a stroke with a fractional width, such as 3.5 pixels—Snap To Pixels snaps to pixel boundaries, and not to the edge of the shape.

To turn pixel snapping on or off, select View > Snapping > Snap To Pixels. If the magnification is set to 400% or higher, a pixel grid is displayed. A check mark appears next to the command when it is on.
To turn pixel snapping on or off temporarily, press the C key. When you release the C key, pixel snapping returns to the state you selected with View > Snapping > Snap To Pixels.
To temporarily hide the pixel grid, press the X key. When you release the X key, the pixel grid reappears.

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