Diacritic
English orthography often uses digraphs (like "ph", "sh", "oo", and "ea") rather than diacritics to show more sounds than can be shown with single letters of the Latin alphabet.  Unlike other systems (such as Spanish) where the spelling shows how to say the words, English pronunciation is so varied that diacritics alone would not make it phonetic. By using digraphs we show sounds which are not shown by single letters.