Every Flash document contains a Stage—on which we place shapes, text, and other visual elements—and a main timeline, through which we define changes to the Stage's contents over time. The Stage (i.e., the main movie) can contain independent submovies, called movie clips (or clips for short). Each movie clip has its own independent timeline and canvas (the Stage is the canvas of the main movie) and can even contain other movie clips. A clip contained within another clip is called a nested clip. A clip that contains another clip is referred to as the nested clip's host clip or parent clip.
A single Flash document can contain a hierarchy of interrelated movie clips. For example, the main movie may contain a mountainous landscape. A separate movie clip containing an animated character can be moved across the landscape to give the illusion that the character is walking. Another movie clip inside the character clip can be used to animate the character's blinking eyes independently. When the independent elements in the cartoon character are played back together, they appear as a single piece of content. Furthermore, each component can react intelligently to the others; we can tell the eyes to blink when the character stops moving or tell the legs to walk when the character starts moving.
ActionScript offers detailed control over movie clips; we can play a clip, stop it, move its playhead within its timeline, programmatically set its properties (such as its size, rotation, transparency level, and position on the Stage) and manipulate it as a true programming object. As a formal component of the ActionScript language, movie clips can be thought of as the raw material used to produce programmatically generated content in Flash. For example, a movie clip can serve as a ball or a paddle in a pong game, as an order form in a catalog web site, or simply as a container for background sounds in an animation.
