The 'replace' edit replaces a clip in the Timeline with an incoming clip, and the 'fit to fill' edit does the same thing, but at the same time, it adjusts the playback speed of the incoming clip so that all of it will fit into the required space [in the Timeline]. Finally there is the 'superimpose' edit, which automatically places the dropped clip on the track above the clip in the Timeline, with a duration equal to the clip below it. Unless an in or out point are set, all edits occur from the position of the playhead in the Timeline. Using the wireframe view on the canvas, the clip can be manipulated directly - dragging it around in the canvas to change its position, for example, or resizing it. Precise adjustment controls for these things are in the viewer. The viewer has tabs for each channel of the selected clip's audio, in which the waveform for the audio can be viewed and scrubbed, and where its volume can be keyframed. The filters tab is where effects for the clip appear and where their parameters can be adjusted and keyframed. If the clip selected is a generator (such as an oval shape), a control tab appears for changing its geometrical properties. Finally, the viewer's motion tab contains tools to adjust the scale, opacity, cropping, rotation, distortion, drop shadow, motion blur and time remapping properties of a clip. Mini-timelines to the right of each parameter allow the property to be keyframed. The Viewer is not present in Final Cut Pro X.  Clips can be edited together in timelines called sequences. Sequences can be nested inside other sequences, so that a filter or transition can be applied to the grouped clips. The Timeline in Final Cut Pro allows 99 video tracks to be layered on top of each other. If a clip is higher [in the timeline] than another, then it obscures whatever is below it. The size of a video clip can be altered, and the clips can be cropped, among many other settings that can be changed. Opacity levels can also be altered, as well as animated over the course of the clip using keyframes, defined either on a graphical overlay, or in the Viewer's 'motion' tab, where precise percentage opacity values can be entered. Final Cut also has more than a dozen common compositing modes that can be applied to clips, such as Add, Subtract, Difference, Screen, Multiply, Overlay, and Travel Matte Luma/Alpha.