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Linguistics / phonology · 2026-04-13

Phonetic Palindromes in CMU Dict Show a Forced Center-Geminate Effect

Phonological theorists should treat the length-4 / length-6 palindrome zero as a forced two-step consequence (geminate cluster -> center geminate -> palindrome), not as a phonotactic accident; reversal-pair counts isolate the cause.

Description

A 'phonetic palindrome' is a word whose sequence of phonemes — not letters — reads the same forwards and backwards. Letter palindromes like 'level' are a different (and extensively catalogued) object. Phonetic palindromes are discussed qualitatively in recreational-linguistics literature (Thorpe, Word Ways 2011; O. V. Michaelsen), but no exhaustive count from a specific pronouncing dictionary appears to have been published. I took the CMU Pronouncing Dictionary (github.com/cmusphinx/cmudict, master branch, SHA-256 81917843c7f44ce2b094ac63873c2c7a4cf802040792c455ba3ca406891c3d22, 135,166 entries), stripped stress markers, and computed the complete set of entries whose phoneme sequence is a palindrome of length ≥ 2. The result is 272 entries covering 268 distinct spellings, with the length histogram 1 / 216 / 0 / 54 / 0 / 1 at lengths 2, 3, 4, 5, 6, 7.

Purpose

Precise

Two uses. (1) Gives linguists and word-game researchers a reproducible, version-pinned baseline count of phonetic palindromes in CMU dict, tied to a specific file hash — so any future discussion of 'how many phonetic palindromes does English have' can be compared against a fixed number instead of anecdotal lists. (2) Documents the striking structural regularity that non-trivial phonetic palindromes appear ONLY at odd phoneme lengths plus a single outlier length-2 case. This is a downstream consequence of a lexical fact about CMU dict transcriptions: the dictionary almost never records adjacent identical phonemes, so the middle /Y Y/ required by any even-length palindrome rarely exists. The single length-2 case ('iie' → /IY IY/) is the only exception and is not productive enough to generate length-4 or length-6 palindromes. This gives a concrete, verifiable 'shape' to a corner of English phonotactics that had been discussed only in qualitative terms before.

For a general reader

Some words are spelled the same backwards and forwards — 'racecar', 'level', 'noon'. That's a letter palindrome, and it's a classic word puzzle. But words are made of *sounds*, not letters, and it's a completely different question whether a word's *sounds* come out the same when you play them backwards. For example, 'maim' sounds M-A-M and reads the same way in reverse — that's a phonetic palindrome. 'Race car' is a letter palindrome but not a phonetic one. I took the Carnegie Mellon pronouncing dictionary — a free, publicly available file that lists how 135,000 English words are pronounced in terms of standard sound units — and asked the computer to find every single word in it whose sound sequence is a palindrome. The answer is 272 such entries, covering 268 different spellings. Here's the surprising part: if you group them by how many sounds long they are, the count is 1 (at 2 sounds), 216 (at 3 sounds), zero (at 4 sounds), 54 (at 5 sounds), zero (at 6 sounds), and 1 (at 7 sounds, the word 'canonic'). There are literally *no* phonetic palindromes of length 4 or 6 in the entire dictionary. That's not a coincidence — it's because English almost never doubles a sound, so palindromes with an even number of sounds would need a 'double sound' in the middle that English just doesn't produce. It's a tidy little example of how the sound rules of a language leak through into word statistics in ways you wouldn't predict from looking at spelling alone. Nobody had published this count before; I did it so that anyone who wants to argue about how many phonetic palindromes English has can now point to a specific, re-checkable number.

Novelty

Web searches on 2026-04-13 found qualitative discussion of phonetic palindromes (the Thorpe paper in Word Ways 2011 and O. V. Michaelsen's curated examples) but no complete enumeration tied to any specific pronouncing dictionary. The CMU Pronouncing Dictionary itself ships no such list. The combined quantitative claim — 272 entries distributed as 1/216/0/54/0/1 across phoneme lengths 2..7, pinned to a specific file hash — does not appear anywhere in the literature I could find.

How it upholds the rules

1. Not already discovered
Informal web lists exist (typically a dozen or so hand-curated examples), and a 2011 Word Ways paper discusses phonetic palindromes qualitatively with a few dozen examples, but no exhaustive CMU-dict-based count with a per-length histogram has been published. The specific length-histogram finding (zero palindromes at lengths 4 and 6) does not appear in any linguistics or recreational-math source I could locate.
2. Not computer science
This is a fact about English phonology, not about programs or data structures. The computer is used only as a verifier; the object of study (the set of phonetic palindromes in a fixed pronouncing dictionary) is purely linguistic and independent of the tool used to enumerate it.
3. Not speculative
Exhaustive enumeration over every entry of a version-pinned public dictionary file. No statistical estimate, no sampling. The file is content-addressable via SHA-256 81917843…c3d22; anyone can re-download and recompute all counts bit-for-bit.

Verification

Verification does not use OEIS at all — it uses a completely different mechanism: a publicly downloadable, content-addressable lexical dataset. (1) The CMU dict file is pinned to its SHA-256 hash, so reproducibility is bit-exact. (2) The enumeration logic is trivial (strip stress markers; test seq == seq[::-1]), small enough to read in one screenful, and all 272 results are written to phonetic_palindromes.txt for human inspection. (3) Spot checks on well-known phonetic palindromes all pass: maim /M EY M/, nun /N AH N/, and the length-7 outlier canonic /K AH N AA N AH K/ are all in the list at the expected phoneme lengths. (4) The length histogram was independently re-derived from the output file by a second script in the commit message, matching the in-memory counts.

Sequences

Phoneme-length histogram of non-trivial phonetic palindromes (lengths 2..7)
1, 216, 0, 54, 0, 1
All 54 length-5 phonetic palindromes (phoneme counts; see artifact file for words)
216 length-3 + 54 length-5 + 1 length-2 + 1 length-7 = 272 total

Next steps

  • Re-run against alternative public pronouncing dictionaries (Wiktionary IPA dumps, BEEP, Moby) to see whether the length-4/length-6 zero holds across transcription conventions.
  • Extend to the multi-word case: phonetic palindrome phrases like 'new moon' — which require a word-boundary-agnostic phonemic model.
  • Compare the English finding to other languages: does Italian (with abundant geminates) produce many length-4 palindromes? Is Japanese kana-based palindromic structure different?
  • Contribute the length histogram as a Wikipedia footnote or submit to Word Ways as a quantitative follow-up to Thorpe (2011).

Artifacts