⟨th⟩ Sounds

The spelling ⟨th⟩ commonly produces 2 sounds in English pronunciation /θ/ and /ð/. Both are fricatives and made with the tongue behind the teeth:

/θ/ is voiceless, it is made only with air, whereas /ð/ is voiced; it is made with vibration of the vocal cords. Some examples of words with each sound are:

/θ/ 
think
bath
author
month

 

/ð/ 
the
this
other
although

Word Groups with ⟨th⟩

Some words spelt with ⟨th⟩ have voiceless /θ/ in noun form, but voiced /ð/ in other forms:

   /θ/                 /ð/
breath (n)  breathe (v)
mouth (n)  mouth (v)
bath (n)   bathe (v)
cloth (n)   clothe (v)
worth (n/adj)  worthy (adj)

Other Pronunciations of ⟨th⟩

Not all words spelt <th> are pronounced with dental fricatives. Some are pronounced with /t/:

Thames /tɛmz/
thyme /tʌɪm/
Thomas /ˈtɒməs/