You can use the word "digraph" in a sentence like this: "A digraph is a combination of two letters representing one sound, such as 'sh' in 'ship'."
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The word digraph is a noun. The plural form is digraphs.
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The antonym for "digraph" is "monograph." While a digraph is a combination of two letters representing a single sound, a monograph is a scholarly piece of writing on a specific subject.