AAAmerican Accent US
Illustration: the history of the word thumb
thumbsilent
A letter that was never said

Why does thumb have a B? It was never said

Unlike lamb and comb, the B in thumb was never pronounced. Old English said þūma — the B was added in the late 1200s by analogy with other -mb words.

Then · Old English

þūma

said like THOO-mah

B added in the late 1200s↓

Now · American English

thumb/θʌm/

said THUM

The story

Thumb looks like lamb and comb, but its history runs the other way. Old English þūma had no B at all — German Daumen and Dutch duim don't have one either. The word probably goes back to a root meaning "to swell" — compare Latin tumēre.

The B started appearing in spellings in the late 1200s. It is what linguists call a parasitic or excrescent letter: after words like lamb and comb had lost their final B in speech, writers had a pattern — "-mb" spelled, "-m" said — and they extended it to words that had only ever had an M. Crumb and limb got the same treatment.

So the B in thumb is silent not because people stopped saying it, but because no one ever did. It is a spelling fashion that stuck — the same kind of thing that happened, centuries later and for different reasons, to the B in debt and doubt.

How the word changed

  1. 1

    Old English (before 1100)

    þūma

    No B. Cousin of German Daumen.

  2. 2

    Middle English, late 1200s

    thoumbe, thumb

    A B appears in writing — never pronounced.

  3. 3

    Modern English

    thumb

    THUM — the B is a spelling ornament.

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Questions people ask

+Was the B in thumb ever pronounced?

No. Old English þūma had no B. It was added to the spelling in the late 1200s by analogy with words like lamb, whose final B had gone silent.

+Why do thumb and crumb have a silent B?

Both are cases of a "parasitic" B: writers added it to match the -mb spelling of words like lamb and comb, though it was never said.

More words with a story

lambLAM

In Old English, lamb was said with a final B. Later Middle English reduced -mb to -m, so lamb now rhymes with ham. Here is the story of the B.

combKOHM

Comb comes from Old English camb, an ancient word for "tooth", and its B was once pronounced. Here is how the B went silent and the vowel moved.

debtDET

English borrowed debt from French as dette, with no B. Scholars added the B later to show off its Latin root, debitum. It was never pronounced.

doubtDOWT

Doubt was spelled doute in Middle English, with no B. Renaissance scholars added one to match Latin dubitare — but English never pronounced it.

Part of

Why does thumb have a B? It was never said