form the plural in a variety of ways. It is customary to classify Swedish nouns into five declensions based on their plural indefinite endings: -or, -ar, -(e)r, -n, and no ending. Nouns of the first declension are all of the common gender (historically feminine). The majority of these nouns end in -a in the singular and replace it with -or in the plural. For example: en ("a girl"), ("girls"). A few nouns of the first declension end in a consonant, such as: en ("a wave"), ("waves"); en ("a rose"), ("roses"). Nouns of the second declension are also of the common gender (historically masculine), with the exception of ett ("a finger"), ("fingers"). They all have the plural ending -ar. Examples include: en ("an arm"), ("arms"); en ("a dog"), ("dogs"); en ("a lake"), ("lakes"); en ("a boy"), ("boys"); en ("an illness"), ("illnesses"); en ("a stranger"), ("strangers"). A few second declension nouns have irregular plural forms, for instance: en ("an evening"), ("evenings"); en ("a summer"), ("summers"); en or en ("a mother"), ("mothers"). The third declension includes both common and neuter nouns. The plural ending for nouns of this declension is -er or, for some nouns ending in a vowel, -r. For example: en ("a park"), ("parks"); ett ("a museum"), ("museums", also loses the Latinate suffix -um); en ("a shoe"), ("shoes"); en ("an enemy"), ("enemies"). Some third declension nouns modify or shorten their stem vowels due to umlaut in the plural: en ("a hand"), ("hands"); ett ("a country"), ("countries"); en ("a book"), ("books"); en ("a nut"), ("nuts"). All nouns in the fourth declension are of the neuter gender and end in a vowel in the singular. Their plural ending is -n. For example: ett ("a bee"), ("bees"); ett ("an apple"), ("apples"). Two nouns in this declension have irregular plural forms: ett ("an eye"), ("eyes"); ett ("an ear"), ("ears"). Fifth declension nouns have no plural ending and they can be of common or neuter gender. Examples of these include: ett ("a child"), ("children"); ett ("an animal"), ("animals"); en ("a teacher"), ("teachers"). Some fifth declension nouns show umlaut in the plural: en ("a mouse"), ("mice"); en ("a goose"), ("geese"); en ("a man"), ("men"). Articles and definite forms The definite article in Swedish is mostly expressed by a suffix on the head noun, while the indefinite article is a separate word preceding the noun. This structure of the articles is shared by the Scandinavian languages. Articles differ