Adobe InDesign User Manual
Page 583
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Note:
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.
After selecting an anchor point (left), direction lines appear on any curved segments connected by the anchor point (right).
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.
In Illustrator, you can show or hide anchor points, direction lines, and direction points by choosing View > Show Edges or View > Hide
Edges.
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