Nouns can be classified as to the form of the endings that can be tacked onto them. For example, in English some nouns take the plural ending -s (cat-s) and some take -es (ditch-es), thus forming two classes of nouns. Russian nouns are classified into four groups, based on the form of the endings that can be added to them. These groups are called declension classes. Some nouns take the Nominative Singular ending -а, some take -о, and some take no vowel ending, or, in other words, zero ending. The names of these three declension classes are: a-declension, o-declension, and zero-declension; the zero-declension is usually written with the cross-hatch symbol: #-declension. The fourth declension is sometimes called the feminine ь-declension. All nouns that belong to it are of the feminine gender; they end in -ь.