Answer: digraphs

AnswerCrossword Clue
digraphsgroup of two successive letters
digraphsDIGRAPH, a pair of letters representation a single speech sound
digraphsa group of two successive letters whose phonetic value is a single sound (as ea in bread or ng in sing) or whose value is not the sum of a value borne by each in other occurrences (as ch in chin where the value is + sh)