But, and it is very important that you understand this, Transitions do not use material (video) from inside the 'in and out' points of a clip they work using material that is outside the 'in and out points'. Transitions work by weaving or blending the end of one clip with the beginning of the next clip. A transition can be modified in many ways and can also be saved as a Favorite for future use. A Transition that is placed at the start of the first clip or the end of the last clip will use the color Black to transition to, a kind of a 'fade to black'.
While transitions are often applied to the edit point between two adjacent clips, they can be applied to the beginning or end of a clip as well or to a single clip, beginning or end. FCP also has many, very graphic transitions like Cube spin, Wipes, Page Peel, etc.
While FCP has some 67 different transitions that we can use, some of the most often used transitions are Cross Dissolve, Dip to Color Dissolve, or Fade in Fade Out Dissolve, which can be subtle. In order to achieve this effect we use Transitions. Often at the end of a scene we want to convey to the viewer a passage of time or place before starting the next scene. This creates the feeling that the scene is happening continuously. When working on a Project in FCP clips from within a scene are often put end to end, what we call straight cuts. The Basics - Editing with Transitions in Final Cut Pro Editing with Transitions in Final Cut Pro