Linear interpolation

Linear interpolation creates a uniform rate of change between keyframes, adding a rhythmic or mechanical look to animations. After Effects interpolates the values to the next keyframe as directly as possible without accounting for the values of other keyframes.

If you apply Linear interpolation to all keyframes of a temporal layer property, change begins instantly at the first keyframe and continues to the next keyframe at a constant speed. At the second keyframe, the rate of change switches immediately to the rate between it and the third keyframe. When the layer reaches the final keyframe value, change stops instantly. In the Value graph, the segment connecting two Linear keyframes appears as a straight line.

If you apply Linear interpolation to all keyframes of a spatial property such as position via the motion path, After Effects creates a straight line between each keyframe. At each Linear keyframe  where a change of direction occurs, the motion path forms a corner. For example, you might use Linear interpolation on the position keyframes of the motion path to create the path of a pinball.