Open and closed syllables: keeping Dutch vowels long or short

Dutch spelling changes a word when you add an ending, so that the vowel keeps sounding the same. That is why maan (moon) becomes manen (moons) but man (man) becomes mannen (men). The trick behind it is the open or closed syllable.

A syllable that ends in a vowel is open, and a single vowel there is long; a syllable that ends in a consonant is closed, and a single vowel there is short. When you add -en, Dutch adjusts the spelling so the vowel stays in the same kind of syllable.

1. Split the word into syllables. A single consonant between two vowels joins the next syllable (ma-nen), while a double consonant is split, one to each syllable (man-nen). (More on this in dividing words into syllables.)
2. If the vowel sits at the end of its syllable (open), it is long: ma-nen — the a is long, so one letter is enough.
3. If a consonant closes the syllable, a single vowel is short: man-nen — the a is short, kept short by doubling the n.

So the same starting letter a can be long or short depending only on what closes the syllable. Nothing about the vowel itself changes — only its surroundings.

A long-vowel word stays long when it grows. Adding -en to maan would give maa-nen, but a syllable never ends in a double vowel, so one a drops: ma-nen. The a is now at the end of its syllable, still long. Going the other way, from the plural bomen (trees) back to the singular, the o would end up in a closed syllable (bom) where a single vowel is short — so you double it to keep it long: boom.

A short-vowel word must also stay short. Adding -en to man would give ma-nen, where the a ends up open and long — the wrong sound. To keep the syllable closed, you double the consonant: man-nen. The extra letter is not pronounced twice; it only shows that the vowel before it is short.

This engine drives most spelling changes in Dutch: the plural -en, verb forms, and adjective endings all lean on it. Which letters you actually write for the long vowel — one or two — is covered in double vs single vowels.

The rule only fires for a single vowel. A fixed vowel combination like oe, eu, ui or ei already counts as one unit and never doubles a following consonant or drops a letter. So deur (door) → deuren, trein (train) → treinen, not deurren or treinnen. Check whether you have one plain vowel or a set combination before applying the rule.