One sound, /k/, and a strict position rule: ck only ever follows a short vowel at the end of a syllable. Never at the start of a word.
Digraphs
ck DIGRAPH_CK_01
Strong rule · High reliability · Very common · Beginner · Reading and spelling
/k/ k: duck, back, rock
Where Wiggler teaches it: Skill 3: Consonant Digraphs
Child explanation
Ck says /k/. It only ever comes right after a short vowel, at the end — like back, sick and duck.
The rule
Ck spells /k/ immediately after a short vowel at the end of a one-syllable word or syllable. It never begins a word and never follows a long vowel, a vowel team or a consonant.
What is really going on
The ck/k/c distribution is one of the most reliable spelling generalisations in English. After a short vowel in a stressed final position: ck. After a long vowel, a vowel team or a consonant: k. At the start of a word before a, o or u: c; before e, i or y: k. For reading, the value of the rule is the reverse inference — seeing ck tells a reader the vowel before it is short.
Position in the word
End of a word — The only position ck occupies. For example: back, duck, rock, sick, neck.
After a vowel — Specifically after a short vowel. For example: pick, luck, sock, black.
When this rule works
The preceding vowel is a single short vowel.
The ck ends a syllable.
When this rule does not work
The vowel is long or a team — that takes k: book, week, look, peek.
A consonant comes before the /k/ — that takes k: milk, bank, silk, think.
The word has more than one syllable and the /k/ is not directly after a stressed short vowel — that usually takes c or ck depending on the word: picnic, panic, traffic.
Made-up words that follow the rule. A child who reads these has the pattern rather than the vocabulary.
nack, flick, prock, bleck
Contrast pairs
back versus bake — ck after a short vowel; k after a long one.
sick versus seek — Same contrast with a vowel team.
pick versus pink — ck after a vowel; k after a consonant.
Decodable sentences
The black duck sat on a rock.
Pick up the stick and check the clock.
My neck is sick and my back is stiff.
Teacher note
This is primarily a spelling rule, but it earns its place in reading because it works backwards: a child who sees ck knows the vowel is short before they have decoded it, which removes a guess. Teach it right after short vowels are secure, and use the two-column sort — bake/back, seek/sick — rather than stating it.