[General] digraph versus diagraph

[sylvester]

What is the difference between a digraph and a diagraph? In a book I'm using, I find:

1. vowel diagraphs ee, ea-- both make the long 'e' sound
2. vowel digraphs ai, ay-- both make the long 'a' sound
3. consonant digraphs th, sh, ch

It seems that all three examples above are showing two independent letters/sounds are joined together to make a unique sound. So why the distinction of "digraph" vs. diagraph?

[Soup]

While <ee>, <ea>, <ai>, and <ay> are digraphs, it appears the author is using the spellings diagraph and digraph to differentiate non-glides and glides. That is, vowel digraphs such as <ai> and <ay> are also called glides.

[konungursvia]

I've never seen [ay] in English. Do they mean [aj]?

[Soup]

Originally Posted by konungursvia

I've never seen [ay] in English. Do they mean [aj]?

Letters, not sounds; e.g., ay as in the word <say>.

[konungursvia]

I've never seen anyone refer to a "diagraph" by the way. But if you're confused about it sly, you might want to consider the origins of the roots in Greek: di, two; dia; through.