Definitions for "Digraphs"
pairs of letters which can occur in the plaintext language (ie English). See here for common digraphs
Two successive letters that make a single sound (e.g., ea in bread or ng in sing).
are graphemes written with more than one letter. Digraphs can represent consonants (ch, sh, th, ck, ng, dge, etc.) or vowels (ai, ay, ee, ea, igh, ew, etc.).