Once children have mastered most of the common sounds we use for single letters, and can blend them together to make words or ‘chunks’, we need to introduce sounds made by more complex letter patterns.
Some sounds are written with 2 letters (digraphs), e.g. consonant sounds th, sh, ch, ph, ck, ng, and vowel sounds: ee, oo, ay, ar etc. Some will use 3 letters (trigraphs) or more, e.g. igh, ore, tch, thr; also ough as in bough or brought. As the original meaning of graph was written (and di was two, tri was three), it’s easy to work out what the words digraph and trigraph represent.
The word diphthong refers to a vowel sound (phthong) where two sounds are blended together. It’s easy to think of ‘ay’, ‘ow’ or even ‘or’ as only one sound, but if you think carefully as you say them (even put your fingers next to your mouth), you will realise that your mouth is actually making one sound then 'gliding' to another sound.
Children enjoy exaggerating the diphthongs so they can hear the two sounds – and it can even help them to spell some, such as ‘ay’ and ‘ow’, when they can hear the /y/ or /w/ sound at the end.
Here is a worksheet to use with children learning ay/ai:
I don’t worry about using the terminology of digraph, trigraph, diphthong etc. with my intervention children and their parents who have enough to cope with, so I just use the loose terms ‘vowel blends’, ‘letter blends’, ‘letter patterns’ etc.
Note - Letter ‘blends’ is also the term commonly used for consonant blends such as bl, cr, st, nt.