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80x24 characters seems to be a very common default for terminal windows. This answer provides a very good historical reason as to why the width is 80 characters.

But why is the height commonly 24 (or 25) lines?

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Save your history questions for next week :P. Too late now though: meta.programmers.stackexchange.com/questions/3546/… –  Dynamic May 15 '12 at 20:28
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80×25 is actually more common –  vartec May 15 '12 at 20:34
    
@vartec : Edited the question for either 24 or 25 lines. –  hotpaw2 May 15 '12 at 20:37
    
I don't know about 25 lines, but whenever I open PuTTY it defaults to 80x24. –  TND May 15 '12 at 20:39
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Probably partly because 80x25 is just under 2K. Makes it easy to have a 2K screen buffer. –  Steven Burnap May 15 '12 at 22:21
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2 Answers

up vote 10 down vote accepted

Early terminals were built around the same cathode ray tubes that were used for televisions. In the 1960's and 1970's these were all 4:3 aspect ratio.

If the display needs to fit 80 characters across the width then given the aspect ratio of the standard characters which was taller than 3:4 (if I remember correctly) and allowing for a larger space between lines than between characters you get to fit 24 or 25 lines on the display.

I haven't done the exact maths because I can't remember (or find) the exact character aspect ratio or line spacing.

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That depends on the bitmap used for the characters. I cannot find an image of the physical screen to see. Also note that 132 character widths were available. –  user1249 May 15 '12 at 22:20
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@ThorbjørnRavnAndersen - I'm having problems finding exact numbers too. If I remember correctly the aspect ratio of the characters used for the 132 character displays were different - thinner than the standard 80 character displays. –  ChrisF May 15 '12 at 22:23
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The height of the vt100 terminal.

Emulators get the best vt100 compatability at that size.

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If so, why did a vt100 display 24 lines? –  hotpaw2 May 15 '12 at 20:39
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Most likely a memory issue. They may have had 1 KB of memory and needed more than 24 bytes for variables. –  user1249 May 15 '12 at 20:49
    
I cannot count. 2 KB. –  user1249 May 15 '12 at 22:48
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