Adobe Flash Professional CS3 - Manual (Page 178)

FLASH CS3
User Guide
172
After selecting an anchor point (left), direction lines appear on any curved segments connected by the anchor point (right).
A smooth point always has two direction lines, which move together as a single, straight unit. When you move a
direction line on a smooth point, the curved segments on both sides of the point are adjusted simultaneously,
maintaining a continuous curve at that anchor point.
In comparison, a corner point can have two, one, or no direction lines, depending on whether it joins two, one, or
no curved segments, respectively. Corner point direction lines maintain the corner by using different angles. When
you move a direction line on a corner point, only the curve on the same side of the point as that direction line is
adjusted.
Adjusting direction lines on a smooth point (left) and a corner point (right)
Direction lines are always tangent to (perpendicular to the radius of) the curve at the anchor points. The angle of
each direction line determines the slope of the curve, and the length of each direction line determines the height, or
depth, of the curve.
Moving and resizing direction lines changes the slope of curves.