Writing Systems of the World 7, Exercise 3: Thai letters, Ge'ez and Devanagari

Why more letters than sounds: the Thai article says tone depends on the
consonant's class (as well as vowel length, the closing consonant and the
tone mark). So extra consonant letters can carry information about tone and
class, not only the sound. (The article's text I read does not give the full
historical reason, so this is the reason the chapter supports, not the whole
story. It is also common for a borrowed script to keep letters for sounds the
borrowing language no longer distinguishes; that is my own general point, not
from the article.)

How Ge'ez and Devanagari differ in showing a vowel: in Devanagari a separate
vowel sign is attached to the consonant (का, कि). In Ge'ez the shape of the
consonant itself is modified, in seven vowel orders (ä, u, i, a, e, ə, o),
and Unicode stores each result as a whole syllable.