Every student of the Russian language knows that Russian nouns have a singular form and a plural form. Many don't know that a thousand years ago those nouns had a “dual form” as well. The dual meant “two of an item”, whereas the plural meant “more than two of an item”. Thus града meant “two cities” and сътѣ meant "two hundreds" and сестрѣ meant “two sisters”. At that time the number дъва was an adjective that agreed with masculine dual nouns and emphasized twoness, and дъвѣ was an adjective that agreed with neuter/feminine nouns and emphasized twoness as well. So back then we had дъва града “two cities”, дъвѣ сътѣ “two hundreds”, and дъвѣ сестрѣ “two sisters”.

Over the centuries time/entropy/life disrupted all that beautiful grammatical symmetry. The "-a" form of masculine nouns often resembled the genitive singular, so nowadays the numbers два/две are followed by nouns in a form that generally coincides with the genitive singular form. The gender association of the numbers shifted as well: nowadays два is used with masculine and neuter nouns, and две is only used with feminine nouns. Here are some sample sentences: