What is the name given to two letters that together form one sound for example ou or ow?

The name given to two letters that together form one sound is called a digraph.

Some examples of digraphs are:

* ou as in "out"

* ow as in "cow"

* sh as in "ship"

* th as in "thin"

* ch as in "chair"

Keep in mind that digraphs can represent different sounds depending on the context. For example, the digraph "th" can represent two different sounds, as in "thin" (voiced /θ/) and "this" (unvoiced /ð/).