digram vs digraph

digram and digraph both are nouns.

Word	Noun	Adjective	Verb	Adverb
digram	Yes	No	No	No
digraph	Yes	No	No	No

As nouns, digram and digraph are synonyms defined as:

digram and digraph: two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')

digram (noun)	digraph (noun)
two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')	two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')