Letters that appear frequently in words. Beginning readers can decode more words when they know several useful letters. Knowing the sounds of /m/, /a/, /t/, and /i/ is more advantageous than the sounds /x/, /q/ /y/, and /z/. Other useful letter sounds are /a/, /e/, /i/, /o/, /u/, /b/, /c/, /d/, /f/, /g/, /h/, /k/, /l/, /n/, /p/, and /r/.