Double vs single vowels in Dutch (aa/a, oo/o)

Dutch spells the same long vowel sound with either one letter or two, depending on the syllable. You write maan (moon) with double aa but manen (moons) with a single a — the sound is identical.

Write a double vowel (aa, ee, oo, uu) for a long sound in a closed syllable — one that ends in a consonant; write a single vowel for the same long sound in an open syllable — one that ends in the vowel itself. A short vowel is always a single letter in a closed syllable.

1. Long vowel, closed syllable (ends in a consonant) → double it: maan, boom, muur (wall), steen (stone).
2. Long vowel, open syllable (ends in the vowel) → single letter: ma-nen, bo-men, mu-ren, ste-nen. A syllable normally does not end in a double vowel (the word-final ee below is the exception).
3. Short vowel → always a single letter, always in a closed syllable: man, bom (bomb), kat, pen.

The five long vowels are aa, ee, oo, uu, and ie. Note that the long i is written ie (dier, animal), not ii, and it does not shorten to a single letter the way the others do. Whether a syllable counts as open or closed is set by the open/closed syllable rule and how you split the word.

There is one place a long vowel keeps its double letter in an open syllable: word-final ee. A single e at the end of a word is the weak uh sound (de, ze, je), so to write a long e sound there you keep both letters: twee (two), zee (sea), idee (idea).

When such a word takes an ending, the double ee is kept and a trema marks the split: zee → zeeën (seas), idee → ideeën (ideas). The other long vowels do not need this: a single a, o or u at the end of a word is already long (na, zo, nu), so there is no weak uh to avoid. Only a single final e turns into a schwa, so only ee is kept double.

A common slip is writing a double vowel in an open syllable, such as maanen or boomen. Native readers catch it at once, because a vowel that closes its own syllable is long on its own — a second letter adds nothing and simply reads as a spelling error. Split the word first: if the vowel ends its syllable (ma-nen, bo-men), one letter is enough.