Consonant gradation: why kenkä becomes kengät

Learn the word kenkä (shoe) and you will meet kengät (shoes) a lesson later. Learn kauppa (shop) and you will find kaupassa (in the shop). Nothing was added or lost by accident: the stem of the word changed. This is consonant gradation, and it is the first thing that makes Finnish look unpredictable to learners. It is actually a rule with very few surprises.

The short version: the consonants k, p and t come in two grades, a strong one and a weak one. Which one you get depends on the syllable that follows.

Strong and weak grade

Take the word kukka (flower). The basic form has a double kk: that is the strong grade. Add an ending like -n and the kk becomes a single k: kukan. That is the weak grade.

The rule is about the syllable that ends up in front of the ending:

Open syllable (ends in a vowel): the stem stays strong. kukkaa, kukkaan, kukkia.
Closed syllable (ends in a consonant): the stem becomes weak. kukan, kukat, kukassa.

You can see which it is by splitting the word into syllables. Ku-kan has the closed syllable kan, so the stem is weak. Kuk-kaa has the open syllable kaa, so the stem is strong. Endings like -n (genitive), -t (plural), -ssa, -lla and -lle close the syllable, so they trigger the weak grade. Partitive -a / -ä and illative with a vowel ending leave it open.

The changes

Strong	Weak	Example (basic form, genitive, partitive)
kk	k	kukka, kukan, kukkaa
pp	p	kauppa, kaupan, kauppaa
tt	t	tyttö, tytön, tyttöä
k	gone	joki, joen, jokea
p	v	leipä, leivän, leipää
t	d	katu, kadun, katua
nk	ng	kenkä, kengän, kenkää
mp	mm	lampi, lammen, lampea
lt	ll	ilta, illan, iltaa
nt	nn	ranta, rannan, rantaa
rt	rr	kerta, kerran, kertaa
lk	l	jalka, jalan, jalkaa

Two groups of changes are worth noticing:

Double consonants lose one. kk, pp, tt become k, p, t. This is the most common change.
A consonant after l, n, r is copied. lt, nt, rt, mp become ll, nn, rr, mm.

Where you will see it

Case endings (see the place cases guide):

kauppa → kaupassa (in the shop), kaupasta (from the shop), but kauppaan (into the shop).
katu → kadulla (on the street).
Helsinki → Helsingissä (in Helsinki).

The plural:

kenkä → kengät (shoes).
kukka → kukat (flowers).
leipä → leivät (loaves).

Verbs have it too. The first and second person (minä, sinä) are weak, and the third person is strong:

Verb	minä (I)	sinä (you)	hän (he/she)
ottaa, take	otan	otat	ottaa
lukea, read	luen	luet	lukee
hakea, get	haen	haet	hakee

When it does not happen

After s. A k, p or t after s does not change: posti (post office) becomes postin, with the t intact.
In many loanwords. Auto becomes auton, and taksi becomes taksin. Newer words often stay strong.
In some other words. Not every word with a k, p or t gradates, and you learn that with the word.

If the word does not gradate, the stem stays the same in every form.

Tips for learning it

Learn two forms of every new word: the basic form and the genitive. Kukka, kukan. From the pair you can see whether the word changes, and how.
Say the weak form aloud. Kaupassa sounds simpler than kauppassa, because it is.
Look at the stem, not the ending. The ending is regular. If a form looks odd, compare its stem with the basic form.

Common mistakes

Keeping the strong grade. Wrong: kauppassa. Right: kaupassa. The ending closes the syllable, so the stem is weak.
Weakening in an open syllable. Wrong: kaupaan. Right: kauppaan. The illative leaves the syllable open, so the stem stays strong.
Weakening k into nothing and losing the word. Joki becomes joen, so the k disappears. Do not look for it.
Over-applying the rule. Words with st, sk or sp do not change.