Tooth

Tooth \TOOTH\

The origin of tooth is Old English toð (plural: teð) from Proto-Germanic *tanth/*tunth, which derived from Proto-Indo-European *dont-/*dent- which meant "tooth."

Tooth/teeth is one of those irregular English quirks that gives second-language learners headaches. It is the result of a type of sound change called 'i-mutation.' In phonetics, vowels are laid out in a map that corresponds to how the sound is made in the mouth.

Vowel Map (source)

 'Front' vowels are made closer to the lips and 'back' vowels are made towards to throat. To illustrate, consider the difference between the 'i' in feet and the 'u' in fudge. Alternate saying both sounds aloud and feel the way your mouth and tongue move for each sound. 'Close' and 'open' vowels have to do with how much space is between the tongue and the roof of the mouth. Alternate saying the vowel sound in feet and fat to feel the difference. I-mutation either moves back vowels to the front or makes front vowels more closed (moves them up), in some cases it does both. The vowel in tooth corresponds to the International Phonetic Alphabet (IPA) symbol [u] while the vowel in teeth corresponds to [i]. From the chart above you can see that the i-mutation involved in tooth/teeth is fronting a back vowel.

So why does i-mutation happen? Vowel changes usually happen to make word easier to pronounce. At some point in Old English toð was pluralized with the suffix -iz. Over time people started 'fronting' the 'o' sound in toðiz more like the 'i' in the suffix because it's easier to pronounce. Seem strange? Say the word doing aloud. Now say How you doing? as you would if you were talking to a friend. Notice how doing sounds a lot like diwin? That is i-mutation. We do it all the time.