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Why "Baths" Doesn't Just Add an /s/ — the Voicing Rule Nobody Explains

Say bath on its own and the final sound is a clean, voiceless /θ/ — the sound in think. Add the plural, and most learners assume they just tack on /s/: /bɑːθs/. But plenty of native speakers actually say /bɑːðz/ — the th itself changes, from voiceless /θ/ to voiced /ð/ (the sound in this), and the /s/ becomes /z/ along with it.

You didn't mishear it, and nobody's being sloppy. This is a real, if patchy, sound change that hits a small set of nouns ending in th when they go plural — and there's no single clean rule that covers every word. You end up learning it in groups.

Group one: words that voice

These end in voiceless /θ/ in the singular, and shift to voiced /ð/ + /z/ in the plural:

What they share: they're all concrete, everyday nouns for things you can point at — a tub, a mouth, a trail, a wreath.

Group two: words that never voice

These stay a clean /θs/ in the plural, no exceptions:

This one isn't random — it's forced by what comes before the th. In these words, a nasal (/n/, /ŋ/) or another consonant sits right in front of the th. Cramming a consonant, then a voiced th, then a voiced /z/ into that small space is awkward to say quickly, so English just keeps the whole cluster voiceless. This "a consonant right before th blocks voicing" pattern is one of the few genuinely reliable rules in this whole topic.

The real trap: words that look identical but don't voice

myth /mɪθ/ → myths /mɪθs/, death /deθ/ → deaths /deθs/, faith /feɪθ/ → faiths /feɪθs/, breath /breθ/ → breaths /breθs/.

Structurally, these are just like bath and mouth — a vowel or diphthong followed by th, nothing in between. Based on group one, you'd expect them to voice too. They don't. This is what makes the pattern genuinely hard: it isn't 100% derivable from spelling. Each word took its own path through the history of the language, and looking similar doesn't guarantee sounding similar.

Even native speakers don't fully agree

Within the voicing group, how strongly the sound actually shifts varies by speaker and region. Dictionaries routinely list two accepted pronunciations for words like paths and truths — both /ðz/ and /θs/ count as standard. If you feel unsure which one is "correct," that's not a gap in your learning. Native speakers haven't fully settled it either.

How to practise it: skip the search for a formula, keep a short list instead

There's no point forcing yourself toward one universal rule — this sound change genuinely doesn't have one. What actually helps is running through the plurals of the high-frequency words — bath, mouth, path, youth, wreath, oath — and fixing their voiced pronunciation in your ear. For anything less common, look it up rather than guessing from a "rule" you half-remember.

Not sure whether a plural should be /θs/ or /ðz/? PHONO reads any English text through your camera and marks every word with IPA, with both US and UK audio to compare, so you don't have to guess.
Download: https://apps.apple.com/us/app/phono-english-phonetics-ipa/id6783243417 · phono.douhouse.com