Locking handles onto the edges of objects
You can easily do this using object snapping.
To lock a handle onto the edge of an object
1. From the Window Menu, choose Object Snapping.
2. Drag the handle near the edge of the object.
If you release the mouse button when the mouse pointer has a small magnet next to it, the handle will rest exactly on the edge of the object.
Tip
This also work with the edges of the page.