When to use Voiced or Voiceless TH/th

When to use voiced or voiceless TH / th

1) In the initial position, the "TH" is voiced in "function words". This includes pronouns, articles, demonstrative adjectives, etc. The list is finite and not very long. They, them, their, theirs, the, this, that, these, those, then, than, though, although, thus, there, (therefore, thereby, etc.)
(Exception =  through)

2) In medial position, the "th" is voiced when followed by "er" or a final silent "e":  feather, mother, brother and breathe, teethe, seethe, writhe, etc. Note: "er" means the spelling, "er", not "or".
"author" is not voiced.   (Also, notice how well the rule works with:
south/ southern, north/northern.)

Exception = Katherine, ether, panther, anther,

3) Words with final thm are voiced: rhythm, algorithm, logarithm.   The following words have a voiceless th, silent th or alternate sound because the thm is not final or the spelling is “them” or “thom.”  voiceless th: arithmetic, anthem, xanthoma, birthmark, bathmat.  Silent th: Isthmian, Isthmus.  Z: asthma.   Exception = fathom (voiced TH)

4) "th" is final position is voiceless with one exception: "smooth".
5) Note:  I've come up with these rules through research and the input of other teachers.  If you find any more exceptions or know of other rules, please share.

Use the rules to determine how to pronounce the “th” in each of these words:

Th and TH list:

Together           Nothing           Thirst               Mother             Thursday

Those               fathom             Everything        These               Thing

There               Authority          Thin                 Then                 Thesaurus

Wrath               Bath                Katherine         ethereal             panther 

anther               algorithm         rhythm              bathmat             With                

Pathetic            Catheter          Faithful             Gothic               Lethal

Mathematics    Seethe             Teethe               Teeth                The     

Bother              Father             Mother             Leather             Feather

North               Northern          South               Southern          Atheist            

Author             Smooth             Myth                Either               Ether               

Isthmus             logarithm