When creating a transition or experimenting with the duration of a video clip, an editor relies on a set of extra frames called edit handles. Edit handles allow the editor an easy way to adjust the clips in his sequence to exactly the right length. Without these handles, the editor would need to manually add more frames from the master clip, or the main body of media, to the subclips created from it.
Edit handles are extra frames of media that exist before and after the visible portion of a subclip. They are composed of the unused portions of the master clip from which the subclip came. Handles exist because digital subclips are not true media; they simply point to a specific section of the media contained in the master clip. Since subclips never exist independently of the master clip, all the frames that exist on the master clip are still connected to the subclip, though they are invisible until an editing operation reveals them.
Handles are used to provide additional frames for transitions and trimming. For example, if an editor wished to put a 24-frame dissolve on the head of a clip and still have the clip begin at the same point as before, he would need 24 extra frames of media. He would get these frames from the handles. Likewise, if the editor decided that the clip was too short and wished to extend the tail end of the clip by using the trim tool, he would need to access the extra frames in the handles.
Handles are available only if extra frames are available on the master clip. To ensure that you will have editing handles, shoot and capture slightly more footage than you would otherwise need. When you create subclips from the masterclip, always place the subclip's "in" point slightly after the beginning of the masterclip and the "out" point slightly before the end. Also, when decomposing media, or reducing the media on your hard drive to only the media used in your project, remember to instruct the system to add handles and enter the number of frames you wish to preserve on either side of the shot in case you need it later for editing.
When handles are purposely created by the editor, their size is often a multiple of the number of frames per second of video. This is usually 24 or 30 in the United States and 25 in Europe. This is because the transitions that commonly use handles are frequently created in half-second or second-long intervals. Creating handles with at least one second's worth of frames ensures that you will have enough frames for a standard transition.