Re: Widths in tables

Greg Kostello (
Mon, 24 Apr 95 17:59:14 EDT

>Dave Ragget writes:
> |> | ch /* characters */
> |
> |> What are "characters" in a proportional-space environment?
> |
> |> In general terms, Jon's inference -- that "ch" is an inappropriate
> |> length measure -- is quite correct. And not only for the reason
> |> he states:
> |
> |I am assuming that when authors choose to use ch that they are aware
> |of the perils involved - it makes perfect sense when the table is
> |in a single fixed pitch font.
> |
> |The use of ems is problematic in this case as an em isn't the same
> |as the width of a character for a fixed pitch font. My understanding
> |is that the em is equal to the point size which is related to the
> |height of the font - not its width.
>That is not the way I heard it. em is a unit of width, not of height
>(ex and others are for height). It is a measure associated with a font
>by the designer and it is traditionally about the width of an M (hence
>the name). An en is exaclty half an em.
>In most fonts (and in fixed width fonts by definition), an en is about
>the width of a digit, so in practice ch can be interpreted to be
>exactly the same as en (or 0.5 em).

The em unit actually refers to the square of the type size. In general,
it is simply a measure of size, and it can be referenced anywhere. For,
example you might choose to set the top margin to 1.5ems or an indent to 2 ems.

Let me quote from Jan Whites Book -- Graphic Design For the Electronic Age:

A square of the type size (or quad) was and continues to be a
proportion. In most typefaces the capital M is shaped more or less
like a square. That is why the square of the type size was nicknamed
an "M." To avoid visual confusion, the "M" is spelled out "em"...
[but this isn't quite the end of the story]

The simplest things tend to be complicated in typography. The
capital M does indeed resemble a square, but the em measures a
little larger than the M. It is the square of the type size: that
measurement must include the descenders of the lowercase letters. In
some faces the ascenders tower above the capital letters, so they,
too, must be included in the overall em's size.

Also from the appendices
Em the square of the type size: in a 10-point type size,
the em measures 10 points high and 10 points wide.

Greg Kostello
Pages Software Inc