The ending -etje is added to nouns that end in a short vowel followed by a single r, l, n, m or ng.
- nouns that end in unstressed -el, -er, or -en get -tje?.
- nouns that end in unstressed -em get -pje?.
- nouns that end in unstressed -ing get -kje?.
Note that the final consonant of the noun is doubled in order to keep the vowel short (see rules for keeping vowels short/long). This does not happen to nouns ending in -ng as the vowel remains short after adding -etje.