Plural of “tooth”

The plural of tooth is teeth.

Why

"Tooth" and "teeth" go back to Old English "toth" and "teth," shaped by the same i-mutation process that produced "foot/feet" and "goose/geese": a plural ending that has since vanished once dragged the root vowel forward in the mouth, and when the ending itself dropped away, the shifted vowel was left behind as the only surviving plural signal.

Dentistry borrowed heavily from Latin and Greek for individual tooth types (incisor, molar, premolar, canine), so none of the technical vocabulary around teeth actually uses the word "tooth" itself in its specialist terms — the irregular plural survives almost entirely in everyday, non-clinical speech about mouths, combs, gears, and saws rather than in the professional vocabulary that describes teeth in fine anatomical detail. The same vowel-shift mechanism is why English never regularized "tooth" toward a modern "tooths" the way some other old nouns eventually drifted toward regular plurals once they left everyday high-frequency use — "tooth" and its plural stayed constantly reinforced in speech from infancy onward, which linguists generally point to as the reason the oldest, most frequent irregular nouns resist regularization far longer than rarer ones. Cross-linguistically, several other Germanic languages preserve a related but not identical vowel-shift plural for their own cognate words, confirming the mutation traces back to a shared ancestor language rather than developing independently within English alone.

Usage note

"Teeth" also extended metaphorically to mean genuine enforcement power ("a law with real teeth") and to describe the jagged edge of tools, combs, gears, keys, and zippers, and in every one of those figurative senses the plural stays "teeth," never a regularized "tooths," even though the object being described (a saw, a comb) has nothing biologically in common with a mouth.

"Teething," the process by which infants' first teeth emerge, is a fully regular gerund built on the noun, and it has its own extended figurative sense ("teething problems" for early setbacks in a new project) that keeps the plural-sounding root but functions as a singular abstract noun in that idiom, distinct from the countable body-part plural "teeth" itself. Dental professionals themselves rarely say "teeth" in clinical charting, instead numbering individual teeth by a standardized system (tooth 1 through 32 in the American system), which sidesteps the plural almost entirely in favor of precise individual identification — a practical workaround in a field where exact reference to one specific tooth among many matters more than describing the set as a whole.

Common real mistakes

"Tooths" shows up occasionally from very young children or learners overapplying the regular -s rule, but it doesn't survive into adult usage the way some irregular plurals partially do; a separate, genuine point of confusion is that "tooth" used adjectivally before a noun stays singular ("a tooth fairy," "a toothpick"), the identical pattern seen in "a six-foot fence." A further subtlety: describing a gear or saw as having "teeths" is a genuine, if rare, hypercorrection error some writers make specifically because they know "tooth" is irregular but overcorrect by adding an extra plural marker onto the already-plural "teeth."

The wider pattern

The same Old English vowel-mutation mechanism behind "foot/feet," "goose/geese," and "mouse/mice" produced "tooth/teeth" — five common nouns explained by one historical sound change rather than five unrelated exceptions.

Examples

• The dentist found a small cavity in one of her back teeth.
• He lost a tooth playing hockey last winter.
• Baby teeth usually start falling out around age six.
• The comb had several teeth missing along one edge.
• The saw's teeth had grown dull after years of use.
• She brushes her teeth twice a day without fail.
• The old gear's teeth were worn smooth from decades of use.
• A wisdom tooth had to be removed last year.
• The zipper's teeth caught on the fabric.
• New regulations finally gave the agency some real teeth.
• The orthodontist adjusted the braces to gradually straighten her teeth.
• He ground his teeth audibly during the tense meeting.
• The mechanic replaced a stripped tooth on the old bicycle gear.