Text sizing¶

Two mechanisms let text occupy something other than one cell per character: the DEC double-width and double-height lines from the VT100, and kitty's per-string text sizing protocol from 2025.

DEC line attributes¶

Status: DEC.

ESC # 3    DECDHL top half of a double-height, double-width line
ESC # 4    DECDHL bottom half
ESC # 5    DECSWL single-width, single-height (default)
ESC # 6    DECDWL double-width, single-height

The attribute applies to the whole current line. A double-width line holds half the columns; text beyond that is discarded. Double height needs two lines carrying the same text, one with ESC # 3 and one with ESC # 4.

The VT100 rendered these with hardware; emulators must scale glyphs. xterm does. Most modern emulators parse and ignore the sequences.

kitty text sizing protocol¶

Status: Vendor-private. Adopters other than kitty: ?.

OSC 66 ; metadata ; text ST

metadata is a comma-separated list of key=value pairs:

Key	Meaning
`s`	Scale: the text is rendered `s` cells tall, 1–7
`w`	Width: the number of cells the text occupies, overriding measurement
`n`, `d`	Fractional scale numerator and denominator for subscript-style sizes
`v`	Vertical alignment within the scaled block: 0 top, 1 bottom, 2 centered
`h`	Horizontal alignment: 0 left, 1 right, 2 centered

The text is at most a small number of code points and cannot contain controls. w alone, with s=1, is the protocol's answer to width skew: the application states how many cells the text should take, and the terminal obeys instead of consulting its own tables.

Terminals that do not implement OSC 66 ignore the whole sequence, including the text, so an application must query first. kitty documents detection via its keyboard or graphics query paths and via TERM_PROGRAM; a generic probe is to send OSC 66 with w=1 and measure with CSI 6 n.

Probe¶

printf '\033#6wide line\033#5\n'
printf '\033#3TALL\n\033#4TALL\n\033#5'
printf '\033]66;s=2;Big\033\\\n\n'
printf '\033]66;w=2;\xe2\x9d\xa4\033\\\033[6n'   # heart forced to width 2

Sources¶

Compatibility¶

Feature support by terminal. Imported claims have not been reverified; unknown is not unsupported.
Feature / stable ID	Alacritty	Apple Terminal	ConPTY (conhost)	Contour	DomTerm	foot	Ghostty	iTerm2	kitty	Konsole	mintty	mlterm	PuTTY	Revenant	RLogin	st	tmux	rxvt-unicode	VTE	wayst	WezTerm	Windows Terminal	xterm	xterm.js
OSC 66 `osc-66-text-sizing`	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	SupportedImported · unverified	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown
DECDWL/DECDHL rendered `esc-double-size-lines`	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	SupportedImported · unverified	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	Unknown	SupportedImported · unverified	Unknown