How it works

Finnish nouns inflect across 15 cases and two numbers — roughly 30 forms per word — with consonant gradation, vowel harmony, and a long tail of irregulars that break any naive lookup table. keinontolibrary produces the forms through several cooperating layers, and then checks every one of them against an independent oracle.

The layers

1. Rule generator (Kotus classes 1–49). Implements the Kotus declension types with consonant gradation (grades A–M) and vowel harmony. It handles the bulk of the language directly — about 98% agreement with the reference corpus.
2. Compound classes (tn 50–51). Compounds inflect through their parts: tn 50 head-inflecting (koirankeksi → koirankeksissä), tn 51 both-parts-inflecting (isoveli → isoissaveljissä). A segmenter splits compounds; a plural-head reverse index and a combining-head registry reach the transparent compounds and derivations that Kotus leaves without a declension type.
3. Corpus-backed lookup. Precomputed surface forms from a reference corpus (collected over three years) are the primary source where available; the rule generator is the fallback.
4. Exception registry + overrides. Genuine irregulars the rules can’t derive — aika → ajan, the suppletive kuka → kenet, vaaka → vaa'an — live in a registry and per-slot overrides.
5. Productive inference & resolvers. Class inference (-nen→38, -uus→40, -ias→41), nested compounds, and compound numerals extend reach to 100% of declinable Kotus nominal rows.

Verification

Correctness isn’t asserted — it’s checked. Every form the engine can produce is validated against Voikko, an open-source Finnish morphological analyzer, used purely as an independent oracle: it analyzes each generated form, and a form is accepted only when Voikko confirms it as the expected lemma + case + number. Where the rule generator and the corpus disagree, the corpus is the witness and Voikko the tie-breaker. The quality gate runs at zero wrong forms across the in-scope inventory.

That is why “verified” is a claim rather than a slogan: each form is checked by a tool that had no hand in generating it.

Data & provenance

• Lemma inventory and inflection metadata — the Kotus Nykysuomen sanalista 2024, licensed CC BY 4.0.
• Surface forms — our own reference corpus, collected over three years and labeled using Voikko as an analysis tool.

Scope: all nominal Kotus classes 1–51 — substantives, adjectives, numerals, and pronouns. Verbs and the indeclinable tn 99/100 classes are out of scope.