Unicode Explained

² is preferred over the superscript two character ² (and its representation

as an entity, ²). This is a debatable issue, partly because superscripting has two

essentially different uses: semantic, as in mathematics, or stylistic, as in abbreviations

like 1

st

for “first” or French M

lle

for “mademoiselle.” This will be discussed in more

detail in Chapter 9.

In practice, compatibility characters are widely used in new Unicode data, too. Many

of them work more reliably than the corresponding “normal” characters. For example,

the micro sign belongs to ISO Latin 1 and therefore appears in almost any font used in

the Western world, whereas the letter mu has less support. Existing software for pro-

cessing measurement data may well recognize “µm” as denoting micrometer but fail to

recognize “μm” (where the letter mu is used).

In using characters, it’s often best to do what everyone else does. Suppose, for example,

that you decide to use the letter mu instead of the micro sign as a unit prefix. If people

open your document in a program and use the program’s search function, the odds are

that they type “µm” using the micro sign. (After all, it’s often easier to write than the

letter mu.) They would not find anything, unless the search function uses advanced

techniques that handle compatibility mappings somehow.

Compatibility Characters for Ligatures

Some compatibility characters have compatibility ...