Indices : Terminology : Digraph

A 'digraph' is the use of two letters to represent a single phoneme or a sequence of phonemes that does not correspond to the letters which compose the digraph. In some languages digraphs are considered individual letters for alphabetizing, casing, &c. In other languages, such as English, this is not the case. Some digraphs have their own code points in Unicode, and as thus it is important to understand which digraphs can be decomposed and which cannot.

Discussion:
Not Ligatures : Digraphs - Lets talk about 'em.
IJ-ligature Fonts

Additional Articles:
Wikipedia: Digraph (Orthography)