The characters which are part of the scripts that are in use all over the world are inventions of mankind. The differences between these scripts and their underlying structures make it plausible that the requirements are mostly culturally and historically based. One cannot apply the same (design) rules regarding harmony and rhythm for the Latin script to Hangul, for example, as the traditional music of Korea is not comparable to Western music and even seems to lack what we call harmony.
Therefore the rules for typography, derive from the scripts, i.e., the letterforms themselves. Rules like the ‘hierarchy of space’ (the relation between the space in the counters, the space between the letters, between the words, between the lines, and the size of the margins) cannot be universally, i.e., ‘cross-scripts’ applied, but only within the (elements of the) scripts themselves. These rules are anchored in what I would like to name grapheme systems (see Systems and Models).
The grapheme systems are the result of the sum of evolution, direct interference of scholars, and (the moments in time of) technical innovations. They are neither perfect nor sacred, but anchored in conventions. Conventions are blueprints for conditioning and conditioning on it’s turn preserves the conventions. That might be a scary thought for those who teach type design (at least it is for me!), but we simply have to acknowledge this fact and deal with it.
To understand the requirements, or to be able to deviate from these, one has to understand the structure of the underlying systems and models. The themes on which all current type designers make variations, ﬁnd their origin in systems and models that were ﬁxed in the ﬁfteenth century by the invention of movable type. Type designers basically put a relatively thin (but often complex) layer of varnish by making variations on the more than ﬁve centuries old themes which have become the standard. The newly created typefaces are the result then of the designers’ insights, technical skills and, of course, the Zeitgeist.
Letterforms created for Latin but outside the conventions, such as Wim Crouwels New Alphabet, can only be judged as such by applying the rules resulting from the underlying structure, i.e., conventions deﬁned by the different type design itself.