Houses, Mouths, Paths: Plurals That Change Sound but Not Spelling

A small group of English nouns change their last sound in the plural even though the spelling does not change: house /haʊs/ becomes houses /ˈhaʊzɪz/, and mouth /maʊθ/ becomes mouths /maʊðz/. The voiceless /s/ or /θ/ of the singular turns into its voiced partner, /z/ or /ð/, before the plural ending. Nothing on the page warns you, so you simply have to know which words do it. The good news is that the list is short, and one word (houses) matters far more than all the others.

You probably already know the regular plural rule: the ending is /s/ after voiceless sounds (cats), /z/ after voiced sounds (dogs), and /ɪz/ after hissing and hushing sounds (buses). If not, start with our guide to the three pronunciations of plural -s. You may also know that some nouns ending in f change both spelling and sound: leaf /lif/ becomes leaves /livz/ (see the f-to-v plural rule). This post covers the hidden version of that same pattern. With s and th, English kept the spelling but, in some words, still changes the sound.

The Rule in Short

• house → houses: always say /ˈhaʊzɪz/, with /z/ in the middle. This also applies to compounds like warehouses and greenhouses.
• mouth → mouths: say /maʊðz/, with a voiced /ð/.
• path, bath, oath, youth, truth, wreath, sheath: the voiced plural /ðz/ is traditional, but many Americans now say /θs/. Both are correct.
• th after a short vowel, a consonant, or r (myths, deaths, months, births, lengths): never voiced. Say /θs/.

Why It Happens: the Logic

This pattern is a fossil from Old English. At that stage of the language, the fricatives f, s, and th each had a voiceless and a voiced pronunciation, but speakers did not treat them as different sounds. The rule was simple: between two voiced sounds (for example between two vowels) the fricative was voiced, and at the end of a word it was voiceless.

Now think about a noun like house. In the singular, the /s/ came at the very end of the word, so it stayed voiceless. The old plural, however, added an ending that began with a vowel, so the consonant ended up between two vowels and became voiced. When English later developed separate /s/ and /z/ (and /θ/ and /ð/) sounds, a handful of common words kept this old alternation. That is why the singular and plural of these words sound different today.

The same history explains several other pairs you already know: knife /naɪf/ and knives /naɪvz/, bath /bæθ/ (noun) and bathe /beɪð/ (verb), and house /haʊs/ (noun) and to house /haʊz/ (verb). If you like this kind of pattern, our post on noun and verb voicing pairs like bath and bathe explores it in detail.

One practical consequence: the voiced plural survives mainly in short, old, everyday words where the s or th follows a long vowel or a diphthong (/aʊ/, /oʊ/, /u/, /i/, and /æ/ in path and bath). Newer words, learned words, and words with a short vowel or consonant before th simply add the regular /s/.

House and Houses: the One to Master

If you remember only one word from this post, make it houses. It is extremely common, and /ˈhaʊsɪz/ with a hissing /s/ is one of the most frequent errors learners make. The standard American pronunciation is /ˈhaʊzɪz/. You will occasionally hear /ˈhaʊsɪz/ from some native speakers, but /ˈhaʊzɪz/ is the norm and the safe choice.

The change carries over to every compound built on house. Notice also the contrast with the possessive house's, which keeps the /s/ of the singular. So houses and house's differ only in /z/ versus /s/.

Be careful not to extend this to other words spelled with -se. Case, place, and piece keep their /s/: cases /ˈkeɪsɪz/, places /ˈpleɪsɪz/, pieces /ˈpisɪz/. The voicing is special to house.

TH Plurals: Mouths, Paths, and Friends

With th, the singular ends in voiceless /θ/ (as in think) and the traditional plural uses voiced /ð/ (as in this) followed by /z/. Here is the core list.

Let's be honest about American usage. For paths, baths, youths, and truths, many Americans today simply add /s/ and say /pæθs/, /bæθs/, /juθs/, /truθs/. Dictionaries list both forms, and both are correct. Mouths /maʊðz/ is the one where the voiced form is still clearly preferred, so learn that one (together with houses) as voiced. For the others, use whichever form feels comfortable, and don't be surprised to hear either.

One bonus detail: the verb to mouth (to form words silently with your lips) is pronounced /maʊð/, with voiced /ð/ even in the base form. It is the same noun/verb pattern as house /haʊs/ and to house /haʊz/.

Exceptions: TH Plurals That Never Voice

How do you spot a word that will not change? Look at the sound right before th. If it is a short vowel, a consonant, or an r, the plural is always a plain /θs/.

• After a short vowel: myths /mɪθs/, deaths /dɛθs/, moths /mɔθs/ (many Americans say /mɑθs/)
• After a consonant: months /mʌnθs/, lengths /lɛŋkθs/, depths /dɛpθs/, widths /wɪdθs/
• After r: births /bɝθs/
• A true exception: faiths /feɪθs/ has a long vowel but is never voiced.

Clusters like /nθs/ in months are hard, and native speakers often simplify them in fast speech. What you should not do is voice these words: /mɪðz/ for myths or /mʌnðz/ for months would sound wrong to an American ear.

How to Pronounce /ðz/

1. Keep your voice on. Start the vowel, and keep your vocal cords buzzing through /ð/ and into /z/. Put your fingers on your throat: you should feel vibration all the way to the end.
2. Tongue movement. Place the tip of your tongue lightly between or just behind your teeth for /ð/, then slide it back to the ridge behind your upper teeth for /z/. It is one smooth movement, not two separate steps.
3. No extra vowel. Mouths is one syllable. Don't say "mouth-es" or "mouths-e".
4. Lengthen the vowel. In English, vowels are longer before voiced consonants. The /aʊ/ in mouths is a little longer than in mouth. Native speakers often partly devoice a final /z/, so this vowel length is an important cue.
5. A native shortcut. In fast, casual speech, many Americans blend /ðz/ toward a simple /z/, so mouths can sound close to [maʊz]. This is acceptable once you can say the full version.

Common Mistakes for Learners

• Saying /ˈhaʊsɪz/ for houses. The most frequent error. Practice the /z/.
• Devoicing the final /z/. Speakers of Spanish (which has no /z/ phoneme) tend to use /s/ for every plural. German speakers carry over final devoicing (Auslautverhärtung), so /z/ becomes /s/ at the end of a word.
• Adding a vowel. Brazilian Portuguese speakers usually find /z/ easy but may add a vowel at the end, as in "mouths-ee".
• Dropping or replacing the ending. French speakers may drop the final consonant, since plural -s is silent in French, or replace th with /s/ or /z/. Then mouths sounds like mouse /maʊs/.
• Overgeneralizing. Don't voice myths, months, deaths, or faiths.

Practice

Listen to each word and repeat it. Pay attention to the voiced /z/ in the house words, the voiced /ðz/ in mouths, paths, baths, and oaths, and the voiceless /θs/ in months and myths.

Sentences to read aloud

1. There are three old houses on our street.
2. The house's roof is newer than the other houses.
3. The new warehouses are next to the old courthouse.
4. The baby birds opened their mouths wide.
5. The two paths meet near the river.
6. They took their oaths in front of the judge.
7. It took months to repair the lighthouses.
8. Many old myths hide simple truths.

Quick Summary

• Some plurals change sound without changing spelling: /s/ → /z/ and /θ/ → /ð/.
• houses /ˈhaʊzɪz/ and all its compounds: always /z/. The possessive house's /ˈhaʊsɪz/ keeps /s/.
• mouths /maʊðz/: learn it as voiced.
• paths, baths, oaths, youths, truths, wreaths: /ðz/ or /θs/, both correct in American English.
• After a short vowel, consonant, or r (myths, months, births): always /θs/. Faiths is an exception.
• The reason: an Old English rule voiced fricatives between vowels, and a few everyday words kept it.