Two rules cover almost everything. First: when a word ends in the voiceless TH /θ/, the plural or third person -s is pronounced /s/ (months = /mʌnθs/). When a word ends in the voiced TH /ð/, the -s is pronounced /z/ (clothes = /kloʊðz/). The ending simply copies the voicing of the sound before it.
Second, and this is the part textbooks leave out: native speakers simplify these clusters all the time, and it is completely acceptable. Months is normally said /mʌns/. Fifths is normally /fɪfs/. Sixths collapses to /sɪks/, identical to six. Clothes is /kloʊz/, identical to the verb close. You are not cutting corners by doing this; you are speaking the way Americans actually speak.
Why these clusters are hard
English allows up to four consonants at the end of a word, and the /θs/ sequence is one of the least comfortable combinations in the language. The tongue has to move from a spot between or just behind the front teeth (for /θ/) to a spot on the alveolar ridge with a narrow groove (for /s/), and it has to do it with no vowel in between. Most languages never ask the tongue to do that, so the reflex is to insert a vowel ("month-es") or to drop the whole thing.
The tongue mechanics for /θs/
Do not think of /θ/ and /s/ as two separate events. Think of one continuous stream of air with a slide in the middle.
- Put your tongue tip lightly against the back of your upper front teeth and start blowing. That is /θ/. Keep blowing.
- Without stopping the air, pull the tongue tip backward and slightly upward until it reaches the bump behind your teeth.
- As the tip retracts, the sides of your tongue seal against your upper molars and the air is forced into a narrow groove. That is /s/.
The whole move takes about a tenth of a second. The single most useful cue: never let the airflow stop. If the air stops, you will either insert a vowel or drop the ending. Practice it as a long hiss that changes color: θθθθsssss. Then shorten it. Then attach it to a word.
For /ðz/ the geometry is identical but everything is voiced. Start humming with the tongue at the teeth, slide back, keep humming. ðððzzzz.
The full decision table
| Word | Careful form | Everyday native form | Note |
|---|---|---|---|
| months | /mʌnθs/ | /mʌns/ | TH stays voiceless, never becomes /ð/ |
| fifths | /fɪfθs/ | /fɪfs/ | TH commonly dropped |
| sixths | /sɪksθs/ | /sɪks/ | Collapses to sound like six |
| twelfths | /twɛlfθs/ | /twɛlfs/ | TH commonly dropped |
| lengths | /lɛŋθs/ | /lɛŋks/ | Many speakers insert a /k/ |
| strengths | /strɛŋθs/ | /strɛŋks/ | Same /k/ insertion |
| depths | /dɛpθs/ | /dɛps/ | TH commonly dropped |
| truths | /truθs/ | /truðz/ | Both forms fully accepted |
| baths | /bæðz/ | /bæðz/ | TH turns voiced in the plural |
| mouths | /maʊðz/ | /maʊðz/ | TH turns voiced in the plural |
| paths | /pæðz/ | /pæðz/ | TH turns voiced in the plural |
| clothes | /kloʊðz/ | /kloʊz/ | Identical to the verb close |
| breathes | /briðz/ | /briðz/ | Verb, always voiced |
Which nouns switch /θ/ to /ð/ in the plural?
Some singular nouns end in voiceless /θ/ but flip to voiced /ð/ when you add -s, which then forces the ending to be /z/. This is the same historical process that gives you knife and knives, wife and wives.
- bath /bæθ/ becomes baths /bæðz/
- mouth /maʊθ/ becomes mouths /maʊðz/
- path /pæθ/ becomes paths /pæðz/
- oath /oʊθ/ becomes oaths /oʊðz/
- youth /juθ/ becomes youths /juðz/ or /juθs/
- truth /truθ/ becomes truths /truðz/ or /truθs/, both correct
And the ones that do not switch, which is where learners most often overcorrect:
- month stays voiceless: months /mʌnθs/, never /mʌnðz/
- length, strength, depth, width, birth, death, myth, faith all stay voiceless: /lɛŋθs/, /strɛŋθs/, /dɛpθs/, /wɪdθs/, /bɜrθs/, /dɛθs/, /mɪθs/, /feɪθs/
- Anything ending in a consonant plus TH stays voiceless. That is the pattern to remember.
The rough generalization: the switch to /ð/ happens after a vowel (bath, mouth, path), not after a consonant (month, length, depth). It is not airtight (births and deaths keep /θ/), but it will get you the right answer most of the time. For the underlying voicing contrast, see our guide to the voiced and voiceless TH rules.
Clothes deserves its own paragraph
Clothes is the single most over-pronounced word in English classrooms. Learners are taught it is /kloʊðz/ and then produce a laborious two-syllable "clo-thez." In real American speech it is one syllable and the TH almost never survives: /kloʊz/, exactly like close the verb. Nobody will misunderstand you, because context does all the work. We cover this in detail in how to pronounce clothes.
Which form should you use?
- Learn the careful form so you know what is underneath and so you can produce it when reading aloud or speaking formally.
- Use the simplified form in normal conversation. It is faster, it is what listeners expect, and it prevents the vowel insertion that actually makes you hard to understand.
- Never insert a vowel. "Month-es" and "six-eth-es" are the errors that genuinely mark you as non-native. Dropping a TH does not.
Practice
Say each word three times: careful, then simplified, then in the sentence. Keep the airflow continuous through the ending.
Drill sentences
- Three months, six months, twelve months.
- Two fifths plus three fifths makes one.
- Fold the clothes and close the drawer.
- Measure the lengths, widths and depths.
- The paths to both baths were blocked.
For more work on stacked endings, see our post on final consonant clusters, then run through the consonant practice exercises to build the muscle memory.