Andrew,

On Tue, Feb 9, 2016 at 6:13 PM, Andrew Bernard [via Lilypond] <
ml-node+s1069038n187064...@n5.nabble.com> wrote:

> What unit is this particular em? In typography at least an em is a
> measurement equal to the currently specified point size for a font. That
> does not seem to fit your analysis here.


This is a good question, and my apologies for just throwing it out
there without defining it properly. I think what you are referring to is a
little different. An "em", like with an "em-dash" is defined as something
that has the same width as the capitol letter "M" in that typeface,
irrespective of pt size. The relative size is the same at 8pt as at 96pt.

What I'm referring to are the units that are used to define the shape of a
glyph, called em-units. These don't have anything to do with the displayed
size. Here's an example. It is fairly common in a text font to make the
capital letter "M" around 800-em units tall. "em-units" are just for
defining a relative scale. As the Wikipedia article[1] states it (under the
"History" section, 2nd paragraph):

"*In digital type, the em is a grid of arbitrary resolution that is used as
the design space of a digital font.*" That's all I mean.

Most fonts are usually defined relative to a resolution of a 1000em-unit
scale, a 1024em-unit scale or a 2048em-unit scale. In virtually all music
fonts, glyphs are defined relative to a 1000-em unit staff-size (center of
bottom line to center of top line), which makes a single staff-space equal
to 250-em units. It's just a convenience thing.

HTH,
Abraham

[1] https://en.wikipedia.org/wiki/Em_%28typography%29




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