A digraph or digram is a pair of characters used in the orthography of a language to write either a single phoneme (distinct sound), or a sequence of phonemes that does not correspond to the normal values of the two characters combined.
Digraphs and trigraphs, groups of characters used to symbolise one character.
Basic latin alphabet has only twenty six letters but most of the languages in this word has more than twenty six sounds.
This is the cause «sh» means [ʃ] and «th» means ð or θ in English.
But there is the second way. Diacritic. is a glyph added to a letter, or basic glyph
You have add a lot of different glyphs to a letter.
There are a lot of combinations. For example, the letter «s» can have the following variations (it is not the full available set)
s̀
ś
ŝ
s̃
s̄
s̅
s̆
ṡ
s̈
s̉
s̊
s̋
š
s̍
s̎
s̏
s̐
s̑
A lot of European languages uses diacritics. But diacritics are rarely used in contemporain English.