Diphthong vs. Digraph

I. Diphthong
A vowel sound that is composed of a sequence of two vowels. The vowel in the word ‘so’, for example begins with the ‘o’ sound of ‘hot’ and then glides into the ‘u’ sound of ‘put’. Other diphthongs are the vowel sounds in the following words:

high  late  toll

Diphthongs should not be confused with digraphs.

II. Digraph
Two letters written together to represent a single sound. For example, these are consonant digraphs:

ch  ck  gh  ph  sh  th

There are also many vowel digraphs in English. For example:

ai  au  ea  ei  oa  oi  ou

For historical reasons, the letters ‘a’ and ‘e’ are frequently joined in the older spelling of words such as medieval:

mediæval

The use of this digraph is, however, dying out.