i3 is a tiling window manager designed for X11, inspired by wmii, and written in C. It supports tiling, stacking, and tabbing layouts, which it handles dynamically. Configuration is achieved via plain text file and extending i3 is possible using its Unix domain socket and JSON based IPC interface from many programming languages.
Like wmii, i3 uses a control system very similar to vi. By default, window focus is controlled by the Alt key plus the right hand homerow keys (Alt+J,K,L,;), while window movement is controlled by the addition of the Shift key (Alt+Shift+J,K,L,;).
The configuration is done via a plain text file, so i3 can be customized without knowledge of programming.
Contrary to other popular tiling window managers, such as dwm, awesome, and xmonad, window management is left to the user in i3. Windows are held inside containers, which can be split vertically or horizontally. They can also optionally be resized. There are also options for stacking the windows, as well as tabbing them (similar to the interface that web browsers now use).
Even though i3 is a tiling window manager, specific windows such as password pop-ups are not displayed as new tiles by default: they are stacked in front of tiled windows. These floating windows can be moved and resized freely, just like in popular desktop environments like GNOME or KDE.
Floating windows are intended by the authors of i3 to be used for pop-up windows only.