You can also create a cross dissolve transition in the timeline without even opening the Transition Window. To do this, select a clip in the timeline that you want to dissolve to and drag it over the clip in front of it. The closer you drag it to the beginning of the first clip, the more gradual the transition between the first clip and the next will be.
A small text box will come up by the mouse, displaying the place in the timeline (in hours:minutes:seconds:hundredths of seconds), the name of the clip you're dragging the transition over, and the length of the transition. You can use this to adjust your transition to the exact length you want.
To get rid of this transition, simply drag the clip in the other direction, and it will go back to its original place in the timeline. It also appears in the Transition track as "Fade," and you can select it in this track and hit Delete as with any other transition.
