You’ve probably noticed that words seem to change shape constantly.

You learn that the word for “flower” is kukka. But then you see someone write “of the flower,” and suddenly it’s kukan.

Where did the other ‘k’ go?

This is called Consonant Gradation (or astevaihtelu in Finnish).

It’s one of the most important grammar rules you’ll learn. It might look hard, but it’s actually quite logical. It exists to make words easier to pronounce.

Specifically, this rule affects the consonants K, P, and T which is why we often call it the “KPT rule.”

In this guide, I’ll break down exactly how and when these letters change.

What is consonant gradation?

Consonant gradation is a sound change that happens inside a word. It affects the “strength” of the consonants k, p, and t when they appear in the last syllable of the word stem.

There are two “grades” (or strengths):

1. Strong Grade: usually the basic form of the word (Nominative).
2. Weak Grade: the modified form when certain endings are added.

Think of it like this: certain endings make the word “heavy,” so the consonants inside need to get “lighter” or softer to make the word flow better.

When does it happen? (The rule of thumb)

The general rule is about open and closed syllables.

• Strong Grade appears when the syllable is open (ends in a vowel).
• Weak Grade appears when the syllable is closed (ends in a consonant).

For example, let’s look at the word katu (street).

If we say “on the street,” we add the ending -lla. The syllable -tu is open (ends in ‘u’), so the ‘t’ stays strong.

Katu → Kadulla. (Wait! actually, -lla creates an open syllable, but standard gradation rules are tricky - let’s stick to the Genitive ending for the clearest example).

Let’s look at the Genitive (possessive) ending -n.

If we add -n to katu, the syllable becomes closed (-tun). Because it ends in a consonant, the ‘t’ becomes weak and turns into a ‘d’.

See the difference? Katu (strong) becomes Kadun (weak).

Double consonants become single

This is the easiest type of consonant gradation to learn. We call this “quantitative gradation.”

If a word has a double letter (kk, pp, tt), it simply drops one letter to become a single letter (k, p, t).

Here is how you might see this in a sentence:

Single consonants change or disappear

This is where things get a little more colorful. Single consonants don’t just disappear; they change into different letters or soften.

The most common change people struggle with is the T → D change, because the sound is very different.

Note on k disappearing:

When k is between two vowels (like in jalka), it usually disappears entirely. However, if the vowels are of the same type (like in sika - pig), the k often disappears but leaves a “long” vowel sound or syllable break.

Special consonant clusters

Sometimes k, p, or t are next to letters like n, l, or r. When this happens, they often assimilate (become like the letter next to them).

This is very common in spoken Finnish and makes the language sound very smooth.

Reverse gradation (Weak to Strong)

Just when you think you understand it, Finnish throws a curveball!

Sometimes, the basic form of the word (the dictionary form) is in the Weak Grade, and when you conjugate it, it becomes Strong.

This happens mostly with:

1. Verb Type 4 (ending in -ata/-ätä) like hypätä (to jump).
2. Verb Type 6 (ending in -eta/-etä) like paeta (to flee).
3. Nouns ending in ‘e’ like osoite (address).

Examples:

• Verb: hypätä (to jump). The basic form has one ‘p’.
  • I jump: Minä hyppään. (Becomes Strong!)
• Noun: osoite (address). The basic form has ‘t’.
  • In the address: Osoitteessa. (Becomes Strong!)

Exceptions

Of course, there are exceptions. The most important one to remember involves the letter S.

If k, p, or t is next to an s, gradation usually does not happen. The s “protects” the consonant.

• Posti (Post/mail) → Postin (Not posdin)
• Lasku (Bill) → Laskun (Not lasun)
• Matka (Trip) → Matkan (However, be careful! tk is not protected by s, but tk generally does not change in standard Finnish).

Also, many proper names do not change, especially first names, though this can vary. Auto (car) is another famous exception - it never changes to audon. It’s always auton.

Summary

Consonant gradation is everywhere in Finnish.

You can’t avoid it, but you don’t need to worry about it.

Here is the quick checklist:

1. Look for K, P, T.
2. Is the syllable open or closed?
3. If closed, reduce the strength (kk → k, t → d, etc.).
4. Watch out for reverse gradation in “e” nouns and specific verbs.

The more you listen to Finnish, the more natural kadun will sound compared to katun. Your ear will tell you what is right before your brain remembers the rule.