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The escape sequences ESC # 3 and ESC # 4 (DECDHL) are meant to be used on pairs on adjacent lines to request double height lines. As an example:

echo -e "\e#3DOUBLE HEIGHT LINE\n\e#4DOUBLE HEIGHT LINE"

Now different terminal emulators behave differently when only a single line is set to double height. xterm happily displays the corresponding half of the double height characters, while konsole < 21.08 displays the whole double height characters.

What did real VTxxx do?

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What did real VTxxx do?

Certainly the VT220 does the same as what xterm does. It just shows the top or bottom half of the character. I don't recall this behaviour depending on the emulation mode.

I'm assuming the other VT's do the same as the VT220. It'd make for much simpler implementation.

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  • 1
    Yes, the VT100 did. Aug 13 at 3:51
  • I have a real VT100, and have written a simulator running the firmware. I can confirm this answer is correct. Sep 2 at 7:29
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Text-based VT-series terminals divide the screen into horizontal stripes whose height is that of a single text line, and generate the video output for each stripe independently. Once a stripe is completed, the rendering hardware will "forget" everything about it as it fetches data describing how to render next stripe.

When rendering text to a bitmap display, as a GUI-based terminal emulator would have to do, it is often easier to draw a line of text without clipping than to show just the top half or just the bottom half. That's probably the reason for the cited behavior. On an actual text-based terminal, however, a line marked as the top or bottom half of a double-height line would be rendered without regard for how any previous lines had been formatted, since taking the formatting of previous lines into consideration would have made things more complicated.

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