Sound-Changing Gradation

Qualitative gradation involves a shift between a strong grade and a weak grade. In most basic nouns and Type 1 verbs, the basic form is strong and the inflected form is weak. However, in inverse gradation (like Type 4 verbs and Type B nouns), this is reversed: the basic form is weak and the inflected form is strong, because the inflected forms take the strong grade when the syllable becomes open (e.g., tavata → tapaan). Note that quantitative gradation (where double consonants like kk, pp, tt simplify to single k, p, t) is the other half of the gradation system.

The Basic Rules:

• k → ∅ (disappears): The 'k' vanishes. Example: jalka → jalan. If 'k' disappears between identical vowels, an apostrophe is used: raaka → raa'an. (Note: ruoka becomes ruoan without an apostrophe because 'o' and 'a' are different vowels).
• p → v: Example: leipä → leivän.
• t → d: Example: katu → kadun.

Consonant Clusters (Assimilation & Others):
When k, p, t follow another consonant, they often assimilate or change:

• nk → ng: Helsinki → Helsingin
• nt → nn: ranta → rannan
• mp → mm: lampi → lammen
• lt → ll: ilta → illan
• rt → rr: parta → parran
• ht → hd: lahti → lahden

Special Cases for k:

• k → v: Occurs specifically in the combinations -uku- and -yky-. Common lexical examples are: suku → suvun, puku → puvun, luku → luvun, kyky → kyvyn, myky → myvyn.
• k → j: The k/j alternation occurs in stems with l or r before k and e after it (e.g., jälki → jäljen, järki → järjen).

Exceptions:
Gradation does not happen in the frozen clusters sk, sp, st, tk. Example: sähkö → sähkön, masto → maston, matka → matkan.

When does it happen?
For standard words, gradation is triggered when a suffix closes the syllable (ends it with a consonant). This happens in the genitive (-n), inessive (-ssa), or adessive (-lla). Illative forms are an important exception to a purely mechanical closed-syllable rule: katuun and Helsinkiin keep the strong grade even though the final syllable ends in n. Learn grade selection with the inflection type and case. In the partitive, Type A nouns stay strong (katu → katua), while Type B words (inverse gradation) use the weak grade (hammasta).

The cluster hk is not generally frozen: nahka → nahan and vihko → vihon lose k, whereas sähkö → sähkön does not. Learn the word’s actual paradigm.

The relevant consonant occurs at the beginning of the final syllable of the inflection stem, after the preceding syllable; inflection does not weaken arbitrary earlier k, p or t sounds. Compound members and derived stems need their own analysis. A noun’s possessive suffix can preserve the strong stem even when the suffix closes a syllable: katumme (our street). An existing case can still select weak grade: kadullamme (on our street).