A digraph is term used to describe two or more letters that make one sound.  Here are some common digraphs:

CH:  pronounced “ch” in cheese, inches, church

SH:  pronounced “sh” in shoe, mission, wish

WH:  pronounced “w” in when, where, why

TH:  pronounced “th” in this, weather, with

PH:  pronounced “f” in photo, hyphen, autograph

TCH:  pronounced “ch: in itch, catch, stretch

WR:  pronounced “r”:  write, wrong, wrench