Key sequence

Press g, let go, then press i, and Gmail goes to the inbox. That is a key sequence: two or more presses that mean something in order, rather than one press with modifiers held down. The distinction is worth keeping sharp, because a chord and a sequence are written almost the same way and typed nothing alike. In a chord the keys are down together and the last one to arrive fires the command, which is what Ctrl plus S is. In a sequence each key is pressed and released on its own, and what the first press does is put the application into a mode where the second press has a different meaning.

The lineage runs through Emacs, which built its whole keymap on prefixes such as C-x, and through vim, where d is an operator waiting for the motion that tells it what to delete. Gmail borrowed the idea for navigation (g then i, g then s, g then t) and a generation of web applications borrowed it from Gmail. The appeal is arithmetic: a modifier based scheme runs out of letters quickly, while one prefix key buys a whole alphabet of commands that need no modifier at all, and the presses stay on the home row instead of becoming hand contortions. That is also the argument for a modifier key never being the only scheme a serious application offers.

The mechanic that has to be designed is the pending state. Between the first press and the second, the application is in a mode nobody asked to be told about, and three things can end it: the expected key, an unexpected key, or time. A timeout is what keeps a stray press from lying in wait, and something on screen has to say that the prefix is held, because a mode with no indicator is a mode the reader can be in by accident. The prefix itself must therefore do nothing on its own, and the escape must be free: Escape cancels, an unrecognized key cancels, and neither costs anything. Sequences also collide badly with typing, so an application that has any text field at all has to stand its sequence handler down whenever the focus is inside one.

Discoverability is the standing weakness, and the answer everyone converged on is a shortcut sheet on ? plus the sequence printed next to the command in menus, which is the same job a command palette does by letting the name be searched instead of memorized. The two are complements rather than rivals: the palette is how a shortcut is found the first time, and the keyboard shortcut is how it is used the hundredth.