English has two TH sounds. Thin /θɪn/ has the voiceless one. This /ðɪs/ has the voiced one. Same letters, different sounds. How do you know which to use? Most ESL textbooks say "memorize the words." But there's actually a rule, and it works almost every time.
The Two TH Sounds
- /θ/ - voiceless: only air, no vibration in the throat. thin, think, math, three.
- /ð/ - voiced: throat buzzes. this, that, mother, breathe.
Touch your throat. If you feel a vibration, it's /ð/. If only air comes out, it's /θ/.
The Three-Part Rule
Use position and word type to predict TH voicing.
1. TH in Function Words = Voiced /ð/
Function words are short words like pronouns, articles, conjunctions. Almost all start with voiced /ð/:
| Word | Type | Sound |
|---|---|---|
| the | article | /ð/ |
| this, that, these, those | demonstratives | /ð/ |
| they, them, their, theirs, themselves | pronouns | /ð/ |
| there, then, than, though, thus | adverbs/conjunctions | /ð/ |
2. TH at the Start of Content Words = Voiceless /θ/
Content words are nouns, verbs, adjectives, and most adverbs. When they start with TH, the sound is voiceless.
| Word | Type | Sound |
|---|---|---|
| thin, thick, thirsty | adjectives | /θ/ |
| think, throw, thank | verbs | /θ/ |
| thumb, throat, thunder | nouns | /θ/ |
| three, thirteen, thirty | numbers | /θ/ |
3. TH at the End of Words = Voiceless /θ/ (Usually)
Final TH is voiceless in nouns and adjectives, voiced in verbs.
| Word | Type | Sound |
|---|---|---|
| bath, breath, cloth, math, north | nouns | /θ/ |
| fifth, ninth, both | numbers/adjectives | /θ/ |
| bathe, breathe, clothe, soothe | verbs | /ð/ |
4. TH Between Vowels = Voiced /ð/ (Usually)
Inside a word, between two vowels, TH is usually voiced.
5. TH Before R = Voiceless /θ/
The cluster THR is always voiceless: three, throw, throat, threat, throughout, threshold.
The Big Exceptions
A handful of words break the pattern. Memorize these:
- thy, thee, thou, thine - old function words → /ð/ (follows the function-word rule)
- booth, with, smooth - these can be either, by speaker preference
- Thomas, Thames, Theresa - some proper nouns just say /t/
- Thai - says /t/, not /θ/
Verb-Noun Pair Pattern
This is the elegant part of the rule. Many noun-verb pairs share a spelling but switch voicing:
| Noun /θ/ | Verb /ð/ |
|---|---|
| bath | bathe |
| breath | breathe |
| cloth | clothe |
| mouth | mouth (as verb: to mouth words) |
| teeth | teethe |
| worth | (none, but "with" is /ð/) |
The verbs voice their TH and often add a silent E. The pattern is part of a wider noun-verb voicing alternation in English (compare house/house, advice/advise).
Quick Decision Flow
- Is the word a pronoun, article, or short connector? → Voiced /ð/.
- Is it the start of a normal noun, verb, or adjective? → Voiceless /θ/.
- Is the TH between vowels inside a word? → Voiced /ð/.
- Is the TH at the very end of a noun? → Voiceless /θ/.
- Is the TH at the end of a verb? → Voiced /ð/.
- Is the TH followed by R? → Voiceless /θ/.
Practice Sentence
Read each TH and decide which sound:
"They thought their mother bathed the three thin brothers in the bath."
- They → /ð/ (pronoun)
- thought → /θ/ (verb start)
- their → /ð/ (pronoun)
- mother → /ð/ (between vowels)
- bathed → /ð/ (verb)
- three → /θ/ (THR cluster)
- thin → /θ/ (adjective start)
- brothers → /ð/ (between vowels)
- bath → /θ/ (noun final)
Once you can predict the voicing, the TH-sound system stops being random and starts being a rule.