Let’s go straight to the point: in short, the em is not a suitable typographic unit for CJK typography because it is related to only one writing direction, but CJK uses two writing directions.
In modern typography, an em simply refers to the point size; but what really is the point size? It has been said that in today’s digital fonts, a point size is nothing other than an arbitrary measure that does not really relate to anything.
However, the em clearly means something: Firstly, in normal proportional fonts, it measures the width of the emdash; and secondly, in old metal terms, it measures the height of the body of the actual metal type. Let’s see how these two meanings apply to CJK fonts.
Let’s first consider the width of the emdash, which leads naturally to two questions: what is the nature of an emdash in a CJK font? and how wide is such a dash in a CJK font? The former is pretty easy to answer — Unicode is pretty clear about it being a fullwidth CJK character —; but the second is a little complicated — it depends on the writing direction.
In practice, most CJK fonts have square glyphs, so the width and height would be the same; but there is no guarantee that this will always be the case, especially since some styles (e.g., Fang Song) naturally lends itself to non-square glyphs. So let’s imagine a hypothetical “condensed font” with a width–height ratio of 950 : 1000.
Suppose we set some hypothetical type, with a Chinese dash (equivalent to a two-em dash), at 21 points. In vertical writing mode, the dash would be exactly 42 points (two ems); however, in horizontal writing mode, the same dash should only be about 40 points, which does not really correspond to anything you can recognize.
So what do these 40 points correspond to? They are clearly two units of something, but of what unit? In common terms (sorry that I don’t know the correct typographical term), this unit has always been just called the “blank space” (空格).
This “blank space” really is just like the em — in that it corresponds to a physical measurement of something in metal type days. What is different is that the “blank space” measures differently depending on the writing mode: in horizontal mode, you measure the width of a CJK fullwidth space, but in vertical mode, you measure the height of a CJK fullwidth space.
So we see that for CJK fonts, the most natural unit is not really the em, but a unit that we can call “the CJK fullwidth space”, which may or may not measure the same in the two writing directions. And this unit is only guaranteed to equal the em in vertical writing mode.
|Example 1: Horizontal writing
|Example 2: Vertical writing