Ordinal numbers are words like first, second, third, fourth, etc. They show position or order of things. If you travel to Russia, it is a good idea to know Russian ordinal numbers. You can see them used when you talk about addresses and in many other situations.

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Unlike English, when talking about years, you cannot say say “nineteen seventeen” or “nineteen ninety one” in Russian. You have to say the whole sequence of numbers, and the last number in that sequence must be ordinal (it is like saying one thousand nine hundred seventeenth year  for “the  year of 1917”).

тысяча девятьсот семнадцатый год  1917
тысяча девятьсот девяносто первый год   1991