Cologne Phonetic (Kölner Phonetik) Algorithm Extraction

Terminology. A phoneme is a contrastive unit of sound. The IPA symbols used below denote: ʃ the "sh" sound (a voiceless post-alveolar fricative) and ts the affricate in German z. Place of articulation is where the vocal tract is constricted; manner of articulation is how airflow is shaped; voicing is whether the vocal folds vibrate. See ../GLOSSARY.md for fuller definitions.

Algorithm Overview

The Cologne Phonetic (Kölner Phonetik) algorithm was developed by Hans Joachim Postel in 1969 for indexing German names. It is more suitable for German than Soundex because it accounts for German pronunciation patterns. Each extracted rule carries a weight in [0, 1] expressing the residual edit cost of treating the two spellings as equivalent.

Encoding Table

Letter	Code	Context
A, E, I, O, U	0	-
B	1	-
P	1	not before H
D, T	2	not before C, S, Z
F, V, W	3	-
P	3	before H
G, K, Q	4	-
C	4	initial before A, H, K, L, O, Q, R, U, X
C	4	before A, H, K, O, Q, U, X (not after S, Z)
X	48	not after C, K, Q
L	5	-
M, N	6	-
R	7	-
S, Z	8	-
C	8	after S, Z
C	8	initial (not before A, H, K, L, O, Q, R, U, X)
D, T	8	before C, S, Z
X	8	after C, K, Q

Extracted Rules

C Context Rules

[id: 2400, name: "c initial before ahkloqrux cologne", weight: 0.10, group: cologne_c]
c -> k / #_[ahkloqrux];  // Cologne, Clara, Christ

[id: 2401, name: "c after sz cologne", weight: 0.10, group: cologne_c]
c -> s / [sz]_;  // SC, ZC combinations

[id: 2402, name: "c before ei cologne", weight: 0.10, group: cologne_c]
c -> ts / _[ei];  // German soft c (Celsius, Cäsar)

[id: 2403, name: "c internal before ahkoqux cologne", weight: 0.10, group: cologne_c]
c -> k / [^sz]_[ahkoqux];  // internal hard c

D/T before sibilants

[id: 2410, name: "dt before c cologne", weight: 0.10, group: cologne_dt]
dt -> ts / _c;  // Stadt→Statc

[id: 2411, name: "ds cologne", weight: 0.10, group: cologne_dt]
ds -> ts;  // Landsmann

[id: 2412, name: "dz cologne", weight: 0.10, group: cologne_dt]
dz -> ts;  // variants

[id: 2413, name: "ts cologne", weight: 0.10, group: cologne_dt]
ts -> ts;  // normalize

X decomposition

[id: 2420, name: "x after ckq cologne", weight: 0.10, group: cologne_x]
x -> s / [ckq]_;  // CKX, KX, QX → same code

[id: 2421, name: "x standalone cologne", weight: 0.10, group: cologne_x]
x -> ks / [^ckq]_;  // AX, EX → 048

PH handling

[id: 2425, name: "ph to f cologne", weight: 0.10, group: cologne_ph]
ph -> f;  // Philipp, Stephan

ST/SP preservation

[id: 2430, name: "st initial cologne", weight: 0.05, group: cologne_clusters]
st -> ʃt / #_;  // German initial st → ʃt

[id: 2431, name: "sp initial cologne", weight: 0.05, group: cologne_clusters]
sp -> ʃp / #_;  // German initial sp → ʃp

V/W equivalence

[id: 2435, name: "v to f cologne", weight: 0.15, group: cologne_vw]
v -> f;  // Vogel (German V)

[id: 2436, name: "w to v cologne", weight: 0.15, group: cologne_vw]
w -> v;  // Wagner (German W)

German-Specific Patterns

Umlauts

[id: 2440, name: "ä to ae cologne", weight: 0.10, group: cologne_umlaut]
ä -> e;  // Bär, Mädchen

[id: 2441, name: "ö to oe cologne", weight: 0.10, group: cologne_umlaut]
ö -> o;  // König, schön

[id: 2442, name: "ü to ue cologne", weight: 0.10, group: cologne_umlaut]
ü -> u;  // München, für

ß handling

[id: 2445, name: "ß to ss cologne", weight: 0.05, group: cologne_eszett]
ß -> ss;  // Straße, groß

Comparison with Soundex

Feature	Soundex	Cologne Phonetic
Target Language	English	German
Code Length	Fixed (4)	Variable
Vowel Handling	Removed	Code 0
Context Sensitivity	None	Yes (C, D, T, X)
Initial Letter	Retained	Encoded

References

1. Postel, Hans Joachim (1969). "Die Kölner Phonetik. Ein Verfahren zur Identifizierung von Personennamen auf der Grundlage der Gestaltanalyse". IBM-Nachrichten 19, pp. 925–931. (No DOI; canonical source of the Kölner Phonetik algorithm.)

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