Many people using Final Cut Express (and possibly Final Cut Pro) have considerable difficulty in getting transitions to work. Dragging a transition to the junction of two clips in the timeline sometimes frustratingly results in a very thin grey representation of the transition (if you are lucky) but no effect in the Canvas. This page is an attempt at overcoming this difficulty by explaining how they work. Understanding how things work is half the battle. Let's start by considering two clips as seen in the Viewer:   
Fig. 1 Outgoing ClipFig. 2 Incoming Clip Note that both clips have their In and Out Points set. This means that the only frames that will be displayed in the Canvas will be those between those points. Any frames outside those points will not be displayed. BUT, they are still there, hidden from the Canvas, because FCE is a Non-destructive Editor. They are hiding in the clips outside the part enclosed by the In and Out Points. Next, imagine that both clips are placed in the timeline next to each other, in the usual way (Fig. 3). The left (earlier) clip is called the Outgoing Clip and the right one (later) is called the Incoming Clip:  Fig. 3 As the timeline's Playhead moves over each frame from left to right, the Project File instructs the Canvas to display each frame. When the playhead reaches the junction of the two clips (where the playhead is shown in Fig. 3) the frame being displayed is the frame which is at the In Point of the Incoming Clip. (Completely ignore the In Point of the Outgoing Clip and Out Point of the Incoming Clip - they are totally irrelevant to this discussion.) Now consider what happens when a Cross Dissolve Transition (one second long - the default) is added to the junction of these two clips (Fig. 4) - half a second on each side of the junction:  Fig. 4 (You might need to read the following paragraphs more than once!) Now, when the playhead is half a second before the junction, (at the left arrow) and while the OutGoing Clip is still being displayed, a frame of the Ingoing Clip must just start to fade in, half a second before the junction and therefore half a second before its In Point. That is what the Cross Dissolve transition requires. But in Fig. 3, where there was no transition, the Incoming Clip did not first appear until the playhead reached its In Point (at the junction). Therefore, FCE must have early access to half a second of Incoming Clip's frames at the same time as those of the OutGoing Clip are fading out. These additional frames (in the Incoming Clip) are new frames, not seen before. Where can these additional frames come from? The answer is: from those frames in the Incoming Clip which are to the left of (which come before) its In Point. And if the Incoming Clip does not have any of these additional frames to offer, the transition cannot work. To put it another way, if the Incoming Clip's In Point is right up against the left end of the clip, the transition cannot work. Tough. Life's a bitch. In the same way, as the playhead moves past the junction, FCE must continue to have access to an additional half a second of the Outgoing Clip's frames at the same time as those of the Incoming Clip, these additional frames gradually fading away completely when the right arrow is reached. These additional frames must come from those frames in the Outgoing Clip which are to the right of (which come after) its Out Point. And if the OutGoing Clip does not have any of these additional frames to offer, the transition cannot work. To put it another way, if the Outgoing Clip's Out Point is right up against the right end of the clip, the transition cannot work. These two sets of additional frames (the ones between the cyan arrows) are called MEDIA HANDLES although the term "Media Handles" is often used to refer also to the Out Point of the Outgoing Clip and the In Point of Incoming Clip if you choose to drag these points (when necessary) to suit a transition. The Transitions Editor in the Viewer shows a useful representation of these handles when the transition is double-clicked in the timeline to get this:  Fig. 5
The two clips are shown as though they were on separate tracks in the timeline and overlapping each other, (which they are not). However, this is quite a good way to imagine what is going on. A transition behaves as if the two clips overlap each other, with each clip, in the overlapped part, being displayed at the same time. Note that the beginning and end of the transition (cyan arrows added by me), are not necessarily at the same places as the Out or In Points, and in Fig. 5 they are not - they are a long way away and not visible in Fig. 5. The Out and In Points can be anywhere you like except that they must not be on any of the frames that the transition will use. If you have a transition that is working OK and you then try to increase its length (from the 1 second in the above example - you can do this in the Viewer (Fig. 5) by typing into the digits in the field at the top left). If the Media Handle for one of the clips is not long enough, FCE will automatically insert the largest workable number. SO, HOW TO SOLVE THE PROBLEM? TUTORIAL You have dragged a transition on to the junction of two clips in the timeline and nothing happens. From the above discussion you will know what the cause is: no media handle in one or both clips. So we must provide them by moving Out and In Points. There are several ways to move In and Out Points. The following paragraphs demonstrates the one that many people use in this situation. Start a new demonstration Project with two clips in the browser and an empty timeline. Double-click one clip to display it in the Viewer and set its In and Out Points at their extreme positions (the In Point fully to the left and the Out Point fully to the right - Fig. 6. (You can drag them in the viewer.):  Fig 6
Drag the clip from the viewer onto V1 in the timeline. This is the Outgoing Clip.. Double-click the other clip in the browser to display it in the Viewer. Again, set its In and Out Points also at their extreme positions (the In Point fully to the left and the Out Point fully to the right.) Drag the clip from the viewer on to V1 in the timeline and position it to the right of the other clip and touching it - the Junction. (It helps to have Snapping turned on in the Timeline.) This is the Incoming Clip. (Fig. 7):  Fig. 7
You have created the conditions causing the problem. Confirm that a transition will not work by dragging a default one-second transition on to the junction of the two clips. (Browser > Effects > Video transitions > Dissolves > Cross Dissolve). You will get an "x" on the mouse pointer. That is the problem. To solve the problem, switch snapping off in the timeline. STEP 1 Choose the Ripple Tool:  Move its pointer very slightly to the left of the Junction (on to the Outgoing Clip) and drag the pointer to the left until the yellow tool-tip reads 13 frames, which is fractionally over half a second (for PAL). This what you should see before releasing the mouse button (Fig. 8):  Fig. 8
and this is what you should see after releasing the mouse button (Fig. 9):  Fig. 9
Note two things. First, you will see that the Junction (and the playhead) has moved half-a-second to the left, caused by the ripple tool shortening the Outgoing Clip. Second, by lining up vertically with your eye the right end of the Incoming Clip in Figs. 8 and 9, it is the whole of the Incoming Clip that has moved 13 frames to the left. This is because the ripple tool automatically closed the gap at the junction which would otherwise be caused by the shortening of the Outgoing Clip. It did this by moving the whole of the Incoming Clip, (but not doing anything else to it). STEP 2 Repeat Step 1 but this time move the pointer very slightly to the right of the Junction (on to the Incoming Clip) and drag the pointer to the right until the time on the yellow tool-tip reads 19 frames, which is a little more then the previous 13 frames. (The reason, for demonstration purposes only, is given later.) (Fig. 10):  Fig. 10
and this is what you see after releasing the mouse button (Fig. 11):  Fig. 11
Note two things. First, the Junction has not moved this time. This is because the Outgoing Clip has not been affected. Second, the Incoming Clip is shorter, and the whole of it has moved to the left (but this time by 19 frames). Again, this is because the ripple tool automatically closed the gap at the junction which would otherwise be caused by the shortening of the Incoming Clip. Now, drag the default Cross-Dissolve Transition on to the junction. Here is the successful result (Fig. 12):  Fig. 12
Step 3. Let's look at the two clips in the viewer - in turn, double-click each clip in the Timeline to see them in the viewer (Figs. 13 & 14):  
Fig. 13 Outgoing Clip Fig. 14 Incoming Clip You will see that the Out Point of the Outgoing Clip has been moved to the left, so making room for the Outgoing Handle; and the In Point of the Incoming Clip has been moved to the right, so making room for the Incoming Handle. Let's look at the transition and part of the clips surrounding the transition as shown in the Transition Editor. Double-click the transition in the timeline and you will get this in the viewer (Fig. 15):  Fig. 15
Compared to Fig. 5, this time the Out and In Points are visible, shown by the change in colour from medium grey to light grey (and by the yellow arrows added by me). They are able to be shown in this figure because the Out and In Points are quite close to the edges of the transition, and therefore they come inside the width of the viewer window. Note also that the In Point of the Incoming Clip is further away from the beginning of the transition than the Out Point of the Outgoing Clip is away from the end of the transition. This is because (for demonstration purposes) the In Point of the Incoming Clip was dragged 19 frames in Fig. 10. The fact mentioned earlier, that the Out and In Points must not be on a frame used by the transition, can be demonstrated by moving the mouse pointer over the transition in the viewer window (it will turn into the Roll tool) and by dragging, trying to move the position of the transition to the left and right. It will be seen that it will refuse to move to a position where an Out or In Point would be inside the transition.  Ripple Training offer a free tutorial concerning transitions, although it does not mention media handles. Navigate: Free Tutorials > Final cut Express > Adding transitions.
 Ken Stone has a more advanced tutorial including more features of the Transition Editor. Navigate: Editing > Editing with Transitions in Final Cut Pro
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