A key frame (or keyframe) in animation an filmmaking is a drawing which defines the starting and ending points of any smooth transition. They are called "frames" because their position in time is measured in frames on a strip of film. A sequence of keyframes defines which movement the spectator will see, whereas the position of the keyframes on the film, video or animation defines the timing of the movement. Because only two or three keyframes over the span of a second does not create the illusion of movement, the remaining frames are filled with inbetweens.