Certain nouns form the plural number with the addition of umlaut to their stem vowel. This is the case with types A, B and C of plural number formation.
| Types of plural | Singular | Plural with umlaut | |
| Type A | ending -e | ||
| Stadt | Städte | ||
| Bach | Bäche | ||
| Knopf | Knöpfe | ||
| Ball | Bälle | ||
| Type B | ending -er | ||
| Haus | Häuser | ||
| Irrtum | Irrtümer | ||
| Bad | Bäder | ||
| Mann | Männer | ||
| Type C | without ending | ||
| Vater | Väter | ||
| Boden | Böden | ||
| Vogel | Vögel | ||
| Mutter | Mütter | ||