This combination of two letters to represent one sound is called a digraph, as gh, in cough, ch in church.
Two letters forming a diphthong or digraph are not to be separated.
When the syllables are divided by the hyphen, there is no hyphen used between the vowels of the digraph.
The digraph "oa" and "ay" may be taught with equal ease the first year.
digraph, dī′graf, n. two letters expressing but one sound, as ph in digraph.
A digraph may either be a combination of two consonants or of two vowels or of a vowel and a consonant.
A consonant or digraph between two sounded vowels usually joins the following vowel, rea-son, no-ti-fy, mo-ther.
When a consonant is doubled (not forming a digraph) the two are generally separated; beg-gar, bril-liant, cun-ning.
The digraph th is represented in Old English texts by and , no consistent distinction being made between them.