Trimming transitions and the surrounding clips, About the two-up display in the canvas, Em on the right (see also – Apple Final Cut Pro 5 User Manual
Page 741: T of the incoming clip on the right (see also, About the, Two-up display in the canvas, P. 406)

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Part III
Fine-Tuning Your Edit
Trimming Transitions and the Surrounding Clips
Trimming is the process of modifying the edit points of clips that are already in your
sequence. After you’ve opened a transition in your sequence in the Transition Editor,
you can fine-tune your edit. This is done using the handles on the Out point of the
outgoing clip and the In point of the incoming clip that meet to form your transition’s
edit point.
Changes you make to a transition in the Transition Editor (if it’s opened from the
sequence in the Timeline and not the Browser) immediately affect this transition in
your sequence.
About the Two-Up Display in the Canvas
You can drag the pointer in the Transition Editor to trim the transition and change the
duration of the transition or the actual location, or edit point, where the transition
occurs between two clips. When you do this, a dual frame display appears in the
Canvas to show how your change affects the clips surrounding this transition.
 The frame on the left: This displays the current frame at the transition’s start point in
the outgoing clip.
 The frame on the right: This displays the current frame at the transition’s end point in
the incoming clip.
Transition start point
Duration of the
incoming clip
In point of the
incoming clip
Duration of the
outgoing clip
Out point of the
outgoing clip
Handle of the
outgoing clip
Transition end point