Snap Alignment
To view alignment guides while positioning elements, choose View > Snapping > Snap Align. With Snap Align active, as you drag elements on the Stage, Flash displays an alignment guide (a dotted line) whenever the element's edge or center gets close to being in alignment with the edge or center of another element, or when the element gets within a user-specified distance of another element's edge or center, or the edge of the Stage (see Figure 2).
Figure 2 Flash's Snap Align feature displays a dotted alignment guide whenever the element you're dragging meets certain conditions.