Consonant gradation (astevaihtelu)

Finnish consonants come in pairs, as a strong and a weak grade: katu, but kadun. Which grade a word shows depends on the syllable. It is easiest to see in the genitive.

The pairs

On the left is the strong grade, on the right the weak grade. The underlined part is what changes. Most nouns show the strong grade in the basic form and the weak grade in the genitive (vowel stems).

Strong / weak	Basic form → genitive	English	More
kk / k	viikko → viikon	week
pp / p	kaappi → kaapin	cupboard	lamppu → lampun (lamp): the triple pp is also shortened to p
tt / t	katto → katon	roof	tyttö → tytön (girl)
k / –	ikä → iän	age	ruoka → ruoan (food): the k simply disappears
k / v	puku → puvun	suit	only between u–u or y–y: luku → luvun (number), kyky → kyvyn
k / j	poika → pojan	boy	after i: aika → ajan (time)
p / v	leipä → leivän	bread	lupa → luvan (permission), apu → avun (help)
t / d	katu → kadun	street	pöytä → pöydän (table), maito → maidon (milk)
nk / ng	kaupunki → kaupungin	town	sänky → sängyn (bed)
mp / mm	lampi → lammen	pond
nt / nn	tunti → tunnin	hour	asunto → asunnon (flat)
lt / ll	ilta → illan	evening	silta → sillan (bridge)
rt / rr	kerta → kerran	time (occasion)
ht / hd	lehti → lehden	leaf	lähtö → lähdön (departure)
lk / l	jalka → jalan	foot, leg	alku → alun (beginning)
rk / r	korko → koron	interest (rate)	päänsärky → päänsäryn (headache)
lk / lj, rk / rj	arki → arjen	everyday life	only before -e-: järki → järjen (reason); jälki → jäljen (trace)
lp / lv	salpa → salvan	bolt
rp / rv	korpi → korven	wilderness

When the weak grade appears

Rule of thumb: a closed syllable has the weak grade, an open one the strong grade. The genitive ending -n closes the last syllable of the stem: ka-tu → ka-dun. So for almost every noun: basic form strong, genitive weak. That is why the stem rules under Vartalo say weak grade for vowel stems.

The other way round for two stem types. Words in -e and -as have the weak grade in the basic form, because there the syllable is already closed by the ending itself. The genitive stem ends in a vowel (-ee-, -aa-), so the syllable becomes open and the strong grade returns: huone → huoneen (nothing to restore), but sade → sateen, hammas → hampaan.

Weak / strong	Basic form → genitive	English	More
k / kk	eläke → eläkkeen	pension	the genitive has the strong grade
t / tt	osoite → osoitteen	address
d / t	sade → sateen	rain	mielipide → mielipiteen (opinion)
nn / nt	liikenne → liikenteen	traffic	luonne → luonteen (character)
hd / ht	lähde → lähteen	source, spring
k / kk	asiakas → asiakkaan	customer
mm / mp	hammas → hampaan	tooth	lammas → lampaan (sheep)
ng / nk	kangas → kankaan	fabric
d / t	tehdas → tehtaan	factory

What has no consonant gradation

Consonant clusters sk, st, sp, tk, hk, ts, ks, ps: keskusta → keskustan (centre), juusto → juuston (cheese), hetki → hetken (moment), sähkö → sähkön (electricity), jakso → jakson (section).
⚠ hk is not reliable: vihko → vihon (exercise book) has gradation, keuhko → keuhkon (lung) does not. Learn vihko as an exception.
After a long vowel or diphthong: tiistai → tiistain (Tuesday), maanantai → maanantain.
One-syllable words: tie → tien (road), työ → työn (work), maa → maan (country).
Single consonants that are not in the table: m, n, l, r, s, h have no gradation (talo → talon, kala → kalan).
⚠ Newer words and loanwords often stay unchanged: pipo → pipon (beanie), auto → auton, tipu → tipun (chick).

How to read the exercise

Look up the stem type under Vartalo.
Is the stem a vowel stem (weak grade) or a stem with strong grade (-e, -as)? That tells you which direction the gradation goes.
Look at the consonant before the last vowel group: does it belong to one of the pairs and not to the clusters without gradation?

Pairs and list of cases without gradation as implemented in tools/fi_stems.py; the examples are dictionary words (salpa, korpi, jälki and kyky are standard Finnish and were added for the rarer pairs).