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**To**:*math-font-discuss@cogs.susx.ac.uk***Subject**:**Re: Big operators (metafontical answer)****From**:*Michael Downes <MJD@MATH.AMS.ORG>***Date**: 18 Aug 1993 08:44:46 -0400 (EDT)

> It is a design feature of the displaystyle operators, that their boxen > extend a bit over the baseline. The ``picture'' does not extend over the > baseline (except for possibly one pixel because of rounding). I don't know > the rationale behind this decision. The mail from Alan, J"org, and Yannis suggests that my previous remarks about the `padded' operator were a big fat red herring and probably totally wrong (not altogether surprising since my impression of the purpose of the padded operator was formed secondhand by overhearing some local Metafont conversations). More likely the true purpose of the padded operator is merely to add the vertical equivalent of side bearings at the top and bottom of the symbols---so that Yannis' proposal to draw boxes around the characters and see how they fit together is after all much to the point. The reason the padded operator is applied only to the displaystyle symbols is then clear: they are the only ones that take limits at top and bottom rather than to the side. I think this leaves unanswered Justin's original question, which boils down (I believe) to: why is the height of textstyle sum zero and depth large, rather than height large and depth zero? (The third possibility, height = 1/2 total + math axis, depth = 1/2 total - math axis, might have been rejected by Knuth in order to avoid building into cmex a dependency on the cmsy value of math axis. But I am not sure there is a great deal of value in this, since extension and symbol fonts probably need to be design-compatible in other respects, e.g. style of braces). Michael Downes mjd@math.ams.org (Internet)

**References**:**Big operators (metafontical answer)***From:*J%org Knappen <Joerg.Knappen@uni-mainz.de>

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