The table gives the characteristics of nouns take - _s_ in the plural.
Nouns that end in a single vowel | Add an apostrophe to nouns ending in single _a, i, o, u,_ or _y_. They need an apostrophe to keep their vowels long.
Do not add an apostrophe to nouns ending in a single e, as this is a [mute e](#/lesson/SpellingAndPronunciation.17).
Exception: adjective-based nouns get _-en_
Nouns that end in unstressed _ee_ | Double _ee_ is a long vowel, the only one allowed in duplo at the end of a word.
Nouns that end in an unstressed vowel combination | The most common unstressed vowel combination you find at the end of a singular noun is _ie_. Other unstressed vowel combinations at the end of a noun are quite rare.
Nouns that end in the following unstressed endings | _-el, -em, -en, -er, -erd, -aar, -aard, -um_
Nouns that end in the following stressed endings | _-eur, -foon_
Person's titles or professions that end in: | _-ier, -oor_
Foreign words that also take -s in their language: | The language has borrowed many words from English and French, where the plural is formed by adding _-s_.
Names of letters and acronyms | Add an apostrophe between the letter/acronym and the _-s_ ending.