23. Many changes of sound, and therefore of writing, occur in the formation and inflection of words. Various causes combine in various ways to produce them; to understand them one must observe the action of the mouth and throat in making speech sounds. The most important relations and changes of Greek sounds are briefly stated in the following sections (§§ 24 – 56).
Meagan Ayer, ed. Goodell’s School Grammar of Attic Greek. Carlisle, Pennsylvania: Dickinson College Commentaries, 2018. ISBN: 978-1-947822-10-8.https://dcc.dickinson.edu/grammar/goodell/sound-changes