Long a inside a word. Pairs with ay, which does the same job at the end — ai almost never ends an English word.
Vowel teams
ai TEAM_AI_01
Strong rule · High reliability · Very common · Early · Reading and spelling
/eɪ/ long a: rain, train, paint
Where Wiggler teaches it: Skill 9: Vowel Teams (Long)
Child explanation
A and i together say /eɪ/, the name of a. You find ai in the middle of words like rain and paint.
The rule
Ai represents long a, /eɪ/, and appears at the beginning or in the middle of a word — never at the end.
What is really going on
Highly consistent for /eɪ/. The distribution with ay is complementary and near-absolute: ai medially, ay finally. The two common breaks are said and again, both extremely frequent function-adjacent words, and the -ain of captain, mountain and certain, where the syllable is unstressed and the vowel reduces to schwa.
Position in the word
Beginning of a word — Reliable. For example: aim, aid, air.
Middle of a word — The usual position. For example: rain, paint, train, chair.
End of a word — Ai does not end English words. Ay does that job.
When this rule does not work
The syllable is unstressed — captain, mountain, certain, bargain all reduce to schwa.
The ai is followed by r, which makes it r-controlled: air, chair, fair, hair.
Made-up words that follow the rule. A child who reads these has the pattern rather than the vocabulary.
blain, shait, fraim
Exceptions
said and again: said, again, against. Two very frequent words where ai is /e/.
Unstressed -ain: captain, mountain, certain, bargain, curtain, villain. Schwa, because the syllable carries no stress.
ai before r: air, chair, fair, hair, pair, stairs. R-controlled, not plain long a.
Contrast pairs
rain versus ran — Team versus single vowel.
rain versus ray — Same sound, different position, different spelling.
main versus mountain — Stressed ai versus unstressed ai.
Decodable sentences
The train waits in the rain.
Paint the nail on the tail of the plane.
I am afraid the main chain is plain.
Teacher note
Teach ai and ay as a pair in one lesson — inside the word, ai; at the end, ay — rather than as two graphemes weeks apart. Children who learn the position rule get both for the price of one and never have to guess which. Name said and again the same day; they are too frequent to leave to chance.