In computing, xterm is the standard terminal emulator for the X Window System. It allows users to run programs which require a command-line interface. If no particular program is specified, xterm runs the user's shell. An X display can show one or more user's xterm windows output at the same time. Each xterm window is a separate process, but all share the same keyboard, taking turns as each xterm process acquires focus. Normally focus switches between X applications as the user moves the pointer (e.g., a mouse cursor) about the screen, but xterm provides options to grab focus (the Secure Keyboard feature) as well as accept input events sent without using the keyboard (the Allow SendEvents feature). Those options have limitations, as discussed in the xterm manual. XTerm originated prior to the X Window System. It was originally written as a stand-alone terminal emulator for the VAXStation 100 (VS100) by Mark Vandevoorde, a student of Jim Gettys, in the summer of 1984, when work on X started. It rapidly became clear that it would be more useful as part of X than as a standalone program, so it was retargeted to X. As Gettys tells the story, "part of why xterm's internals are so horrifying is that it was originally intended that a single process be able to drive multiple VS100 displays." After many years as part of the X reference implementation, around 1996 the main line of development then shifted to XFree86 (which itself forked from X11R6.3), and it is now maintained by Thomas Dickey. Many xterm variants are also available. Most terminal emulators for X started as variations on xterm. Features Terminal emulation Early versions emulated the VT102 and Tektronix 4014. Later versions added control sequences for DEC and other terminals such as: VT220: Added in patch 24. Later, in 1998, xterm added support for VT220 features, such as extending its support of ISO-2022 shift functions to provide the National Replacement Character Set feature. VT320: Added in patch 24. VT420: DECSTR (soft terminal reset) was added in patch 34. In 2012, xterm's default emulation was changed to VT420 to allow tmux to assume the corresponding left/right margin support. VT520: Although not officially emulated, parts of VT520 features were implemented. Controls DECSMBV and DECSWBV for setting the margin- and warning-bell volume were added in patch 254. Customization As with most X applications, xterm can be customized via global X