Follow-through

Follow-through and overlapping action are the fifth of Disney’s twelve principles, and they describe one observation: nothing made of parts starts or stops all at once. A runner halts and the coat keeps going for a beat. An arm swings and the hand arrives after the elbow. The body leads, the loose things trail, and the picture reads as a connected object rather than as a shape being repositioned. In an interface the loose things are a badge on a card, an avatar, a caption line, a shadow, an icon inside a button.

The trick is small offsets, not different destinations. Every part travels to the same place; each one simply leaves and lands on its own beat, usually 40 to 120 milliseconds behind the body, often with a hair of correction at the end. Larger than that and the card looks broken into pieces; smaller and nobody feels it. This completes a trio the other two members of which are already words: anticipation is the small move the wrong way before the main one, overshoot is the surplus carried past the target, and follow-through is what the trailing parts do after the leader has stopped.

It is worth separating from two neighbours. Stagger offsets peers, a list of rows entering one after another, where the items are unrelated to each other; follow-through offsets parts of a single object, where the lag is what says they are attached. Secondary action, Disney’s principle seven, is a different gesture entirely: a second motion that adds meaning rather than a delayed copy of the first. If the parts are moved by a spring rather than by timed curves, the same effect falls out of giving each part slightly less stiffness than its parent, which is why gesture-driven interfaces get it almost for free.

The costs are the usual ones. Every offset extends the total time the reader waits before the composition is at rest, so keep follow-through for arrivals worth watching and off anything triggered many times an hour. Text should not trail, because a line still sliding is a line that cannot be read. Nothing that trails may change layout, since a part still settling is a part still pushing its neighbours around. And under prefers-reduced-motion the whole composition simply arrives, together, at rest.