# BiDi Modes (?1243, ?2500, ?2501) > Swap the arrow keys in right-to-left text, mirror box drawing, and detect the direction of each paragraph, for a terminal that handles bidirectional text. - **Sequence:** `CSI ? 1243 h/l` - **?2500**, Box Drawing Mirroring: `CSI ? 2500 h/l` - **?2501**, BiDi Autodetection: `CSI ? 2501 h/l` - **Specification:** [BiDi in Terminal Emulators, v0.1, The escape sequences](https://gitlab.freedesktop.org/terminal-wg/bidi/-/blob/cc8686a93c8cc2f25a2f68f79c01b4822111509c/docs/30-recommendation/90-escape-sequences.md) [*BiDi in Terminal Emulators*](https://terminal-wg.pages.freedesktop.org/bidi/) is Egmont Koblinger's recommendation for how a terminal should show text that runs right to left, such as Arabic and Hebrew, and text that mixes the two directions. Version 0.1 was published in January 2019. Its [escape sequences](https://terminal-wg.pages.freedesktop.org/bidi/recommendation/escape-sequences.html) are two from ECMA-48, [BDSM](https://control-codes.page/ansi/bdsm/index.html.md) and [SCP](https://control-codes.page/csi/scp/index.html.md), and the three private modes on this page, which ECMA-48 has nothing for. The recommendation notes that their numbers are VTE's work in progress and not meant to be final; VTE added them in 0.58, in 2019, and still uses them. | Mode | Set | Reset | Default | | --- | --- | --- | --- | | `?1243` | the Left and Right arrow keys are swapped while the cursor is in a right-to-left paragraph | the arrow keys are never swapped | set | | `?2500` | box drawing characters are mirrored in right-to-left text | they are drawn as they are | reset | | `?2501` | each paragraph's direction is detected from its text, falling back on the one SCP set | the direction is the one SCP set | reset | The numbers are VTE's. 2500 and 2501 borrow U+2500, where Unicode's box drawing characters begin, which the recommendation calls "a total misuse of the hexadecimal". 1243 is the permutation applied to the four arrow keys taken in the order of the sequences they send, up, down, right and left: the first two stay put and the last two change places. BiDi is display only. The terminal keeps the text in the order it arrived, and the cursor addresses cells in that order, so these modes change what the user sees and what the arrow keys send, and nothing that DECRQCRA or a cursor report can read back. ## The modes **Arrow key swapping, `?1243`.** Text in a right-to-left paragraph runs leftwards, so a user who presses Left means to move forwards through it, while the program reads Left as backwards and moves the cursor the other way on the screen. With the mode set, the terminal sends Right's sequence for the Left key and Left's for the Right key while the cursor is in a right-to-left paragraph, an autodetected one included. It is the only one of the five settings that is global rather than kept per paragraph, as every other mode that affects the keyboard is, which is the recommendation's reason for making it a DEC private mode. It is set by default, matching BDSM, so that a program that does nothing gets keys that move the way they point. **Box drawing mirroring, `?2500`.** Unicode already mirrors characters such as brackets in right-to-left text. Set, this mode mirrors the box drawing characters too, so that a frame drawn by a program that thinks left to right still joins up when it is shown the other way round. mintty also mirrors the Block Elements, U+2580 to U+259F. **Autodetection, `?2501`.** In implicit mode, set, the terminal picks each paragraph's direction from its first strong character, the way the Unicode Bidirectional Algorithm does, and falls back on the direction SCP set when there is none. The recommendation makes reset the default with some hesitation: autodetection is not in ECMA-48, it gives fewer surprises to someone who never uses right-to-left text, and it fixes many cases without fixing all of them. In right-to-left mode, `cat /etc/services` comes out mostly right with it, but with a lone `#` still on the wrong side. A program that needs one or the other saves the mode, sets it, and restores it on exit. The recommendation has no sequence for the level of conformance, such as whether a terminal remembers BiDi control characters; a terminal always uses the highest level it implements. ## How the sequences apply The recommendation marks this part as a draft. Arrow key swapping is global and takes effect at once. The other four settings, BDSM, SCP, box drawing mirroring and autodetection, are kept for each *paragraph*, the run of rows from one hard newline to the next, and the terminal also has a *current* value of each, the one the latest sequence set. A sequence on its own changes nothing visible, any more than `CSI 31 m` on its own makes anything red. - A sequence always sets the current value. If the cursor is at the first position of a paragraph, the value is applied to that paragraph as well, even if it is already the current one, and the paragraph's other settings stay as they are. Anywhere else, the start of the paragraph's second row included, the paragraph keeps what it had. - A newline that takes the cursor into a new paragraph gives that paragraph the current values. One that stays in the same paragraph does nothing. - When text wraps and joins two paragraphs, the joined one keeps the first paragraph's settings. - A new line scrolled in at the bottom, other than by a wrap, gets the current values. - [ED](https://control-codes.page/csi/ed/index.html.md) makes each line it erases a paragraph of its own, with the current values: `ED 0` the lines below the cursor's, `ED 1` the lines above it, and `ED 2` every line, the scrollback excluded each time. The line between the scrollback and the screen becomes a hard newline too, even for an `ED 1` on the top row that erases nothing. This is how a full-screen program, which clears the screen with ED when it starts, gets its modes onto every line. - [SU](https://control-codes.page/csi/su/index.html.md) and [SD](https://control-codes.page/csi/sd/index.html.md) make every boundary where lines are removed or inserted a hard newline, give each new blank line the current values as a paragraph of its own, and move each paragraph's settings with its text. - When [DCH](https://control-codes.page/csi/dch/index.html.md), [EL](https://control-codes.page/csi/el/index.html.md) or the like splits a paragraph in two, both halves keep its settings. [VTE](https://gitlab.gnome.org/GNOME/vte/-/blob/0.84.1/src/vte.cc#L3717-L3744) applies a sequence to the paragraph when the cursor is in its first column and the row before did not soft-wrap into it. ## Implementations [VTE](https://gitlab.gnome.org/GNOME/vte/-/blob/0.84.1/src/modes.py#L1209-L1235) implements all three, with the defaults above, and answers [DECRQM](https://control-codes.page/csi/decrqm/index.html.md) for them with `1` or `2`. It honors arrow key swapping only while BiDi is enabled in the widget. mintty implements `?2500` and `?2501`, not `?1243`. Its autodetection is set by default, where the recommendation and VTE leave it reset ([documentation](https://github.com/mintty/mintty/wiki/CtrlSeqs#bidirectional-rendering)). It also has modes of its own to turn BiDi off, `?77096` for everything and `?7796` for the current line. A default xterm knows none of the three. It ignores them, and answers DECRQM with `0`, not recognized, as it does for any mode it does not know ([`do_dec_rqm`](https://github.com/ThomasDickey/xterm-snapshots/blob/xterm-411a/misc.c#L5466)). libghostty-vt does the same, and the cases follow xterm. ## Support | Terminal | Version | Support | Notes | | --- | --- | --- | --- | | VT220 | [manual](https://vt100.net/docs/vt220-rm/), 2nd ed., 1984 | No | not in the manual | | VT510 | [manual](https://vt100.net/docs/vt510-rm/), 1st ed., 1993 | No | not in the manual | | xterm | patch 412, [xterm-411a](https://github.com/ThomasDickey/xterm-snapshots/tree/xterm-411a) | No | not recognized; DECRQM reports `0` | | libghostty-vt | [919dca5](https://github.com/ghostty-org/ghostty/tree/919dca5c848d43a489a4cb2e1feade016f0fa2db) | No | not recognized; DECRQM reports `0` | | ConEmu | build 230724, its [documentation](https://conemu.github.io/en/AnsiEscapeCodes.html) | No | not in its [ANSI escape codes](https://conemu.github.io/en/AnsiEscapeCodes.html) | | mintty | 3.8.3, its [documentation](https://github.com/mintty/mintty/wiki/CtrlSeqs) | Partly | `?2500` and `?2501`, with autodetection on by default; no `?1243` ([documentation](https://github.com/mintty/mintty/wiki/CtrlSeqs#bidirectional-rendering)) | | VTE | 0.84.1, [source](https://gitlab.gnome.org/GNOME/vte/-/tree/0.84.1) | Yes | all three, with the BiDi recommendation's defaults ([source](https://gitlab.gnome.org/GNOME/vte/-/blob/0.84.1/src/modes.py#L1209-L1235)) | ## Validation ### BIDI-1: A default xterm does not recognize them Input, one step per line: ```text \e[?1243$p \e[?2500$p \e[?2501$p ``` Expected screen: ```text |____| reply \e[?1243;0$y\e[?2500;0$y\e[?2501;0$y ``` ### BIDI-2: Nor after they are set Input, one step per line: ```text \e[?1243h\e[?2500h\e[?2501h \e[?1243$p \e[?2500$p \e[?2501$p ``` Expected screen: ```text |____| reply \e[?1243;0$y\e[?2500;0$y\e[?2501;0$y ``` ### BIDI-3: Box drawing is stored as it arrived Input, one step per line: ```text \e[?2500h \u{250C}\u{2500}\u{2510} ``` Expected screen: ```text |┌─┐_| cursor 1,4 ``` --- This is the Markdown version of . On that page every validation case runs live in libghostty-vt, the terminal emulation core of Ghostty, compiled to WebAssembly. The validation cases are written in the notation described in .