ch chop

ch can start or end a word (chop, such)

use ch when preceded by a consonant (eg. ranch) or a vowel team (eg. peach)

tch catch

tch is only at the end of a word (catch, watch).

Use the longer spelling, tch, after a short vowel (catch).

ch is two consonants (digraph), one sound /ch/

tch is three consonants (trigraph), one sound /ch/