This tool checks Finnish words for vowel harmony, the rule that governs which vowels — and therefore which suffixes — can appear together in a single word.
How it works
Finnish divides vowels into three groups:
back : a, o, u
front : ä, ö, y
neutral : e, i (compatible with either group)In a native, non-compound word, back and front vowels cannot mix. The checker
scans each word, records whether it contains any back vowel and any front vowel,
and flags the word when it contains both. Words with only one group, or only
neutral vowels, pass. This matters because suffixes harmonise with the stem: a
back stem takes -ssa (talossa) while a front stem takes -ssä (metsässä).
Vowel harmony and suffix choice — the practical use case
The core reason Finnish learners and language tools need vowel harmony checking is suffix selection. Every grammatical suffix in Finnish comes in two forms, one for back-vowel stems and one for front-vowel stems. Getting this wrong produces ungrammatical forms that a native speaker will notice immediately.
| Case | Back-vowel suffix | Front-vowel suffix | Example (back) | Example (front) |
|---|---|---|---|---|
| Inessive (in) | -ssa | -ssä | talossa | metsässä |
| Elative (from in) | -sta | -stä | talosta | metsästä |
| Illative (into) | -(h)an | -(h)än | taloon | metsään |
| Adessive (at/on) | -lla | -llä | talolla | metsällä |
| Allative (to) | -lle | -lle | (same) | (same) |
| Partitive | -a / -ta | -ä / -tä | taloa | metsää |
| Translative (becoming) | -ksi | -ksi | (same) | (same) |
Suffixes that are identical in both groups (allative, translative) are not affected by harmony. But most common cases require the right variant, so checking which group a stem belongs to is the first step in conjugating or declining any Finnish noun.
Words with only neutral vowels
Some Finnish words contain only e and i with no back or front vowels — for instance meri (sea), kivi (stone), or piste (point). These stems are neutral and by convention take front-vowel suffixes:
meressä(in the sea) — not*meressävs*meressa; front form is conventionalkivellä(on the stone) — front suffixllä, notlla
The checker labels these words as neutral rather than back or front, which is the correct classification.
When a flag does not mean an error
The checker reports mixing as a flag rather than an error because legitimate cases exist:
Compound words. Finnish builds compound nouns by joining stems, and each component keeps its own harmony. syyskuu (autumn month) = syys (front) + kuu (back). The checker flags this, but the word is correct. Suffixes harmonise with the last element of the compound: syyskuussa (in September) takes the back -ssa because kuu ends in a back vowel.
Loan words. Many loan words violate harmony because they were borrowed as-is: olympialaiset, bussi, futis. These are standard Finnish but mix vowel groups. The checker flags them to alert you, not to call them wrong.
Names and proper nouns. Finnish names from Swedish, Latin, or other sources frequently mix vowel groups. Helsinki, for example, is treated as a back-vowel stem by convention (Helsingissä), even though it contains front vowels.
Practical tips
- Use the harmony check before adding a suffix to an unfamiliar word, especially a compound or borrowed term.
- If the checker flags a word you believe is correct, it almost certainly is — compounds and loans are the usual reason.
- For compound words, identify the final element and check its harmony to determine which suffix variant to use.
- All analysis runs locally in your browser — your words never leave your device.