Directionality

A screen cannot arrive without arriving from somewhere. Every transition already carries a direction whether or not anyone chose it, and directionality is the decision to make that direction mean something: deeper into the hierarchy comes in from one side, back out comes from the other, a sibling slides across, a modal rises from the bottom. Do it consistently and the interface acquires a geography. The reader stops asking where they are, because the motion has been telling them all along, and the back gesture feels like retracing steps rather than issuing a command.

The rule is cheap to state and easy to break. Direction is a property of the relationship between two views, not of the control that was pressed, so every route to a screen has to move the same way. A detail view reached from a list, from a search result, and from a notification all sit in the same place in the hierarchy, and if one of them slides in from the left because that control happened to be on the left, the geography is now a lie. It also has to be mirrored for right-to-left reading, so “forward” is an inline-start to inline-end idea rather than a literal left and right (see RTL mirroring). And it does not need much: a push transition between hierarchy levels and a shared axis move between peers cover most of an application, and the easing matters less than the consistency.

This is exactly where it must not be confused with origin-aware animation, which is the other way motion can carry meaning and is a different claim. An origin-aware transition starts from the control that triggered it, so a menu grows out of its own button and a card expands out of the thumbnail that was tapped; the direction is local, personal to that one gesture, and it changes with wherever the control happened to sit. Directionality encodes where you are in a hierarchy, so it is the same whichever control you used and it is meaningless without the rest of the system agreeing with it. Both can run at once in the same interface, and they answer different questions: one says what you came from, the other says where you have gone.

The most convincing evidence for the rule is what happens without it. Animate every navigation the same way, in from the right whatever the reader did, and going back feels like going forward again. Nothing looks broken and nothing is broken, but the reader loses their sense of place and starts checking the header to work out where they ended up. That is the cost of undirected motion, and it is also the reason a directionally consistent interface can get away with much subtler transitions than an inconsistent one.