A single letter cannot represent very different sounds in different contexts, e.g. Spanish is more transparent in its sound-orthography mapping than English

Columbia University

Types of mapping letters of language orthography and phones

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

In an opaque mapping between the letters of orthography and phones, a singular letter can represent different sounds in different contexts, e.g. sound-orthography mapping in English

e.g. The English letter c corresponds to phone [k] in con [k aa n], but phone [s] in cent [s eh n t]

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