In English, cardinal numbers (for example, "one," "two," or "three") refer to quantity, whereas ordinal numbers ("first," "second," or "third") refer to distribution. Ordinal numbers are used in dates and fractions. They are employed as adjectives to describe importance, position in a list, and placement in time.

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Generally, ordinal numbers are created by adding -th to the end of the cardinal number. However, there are exceptions for the numbers one (first), two (second), three (third), five (fifth), eight (eighth), nine (ninth), and twelve (twelfth). 

I've been nervous. I think you know it's my first video ever.

Caption 31, Adele - The Making of 'Chasing Pavements'

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It is the world's sixth largest country by total area.

Caption 3, Soccer World Cup - Australia

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On the twelfth day after Christmas, we have to take down all the decorations and the tree.

Caption 47, Christmas traditions - in the UK

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Starting with twenty, the ordinal numbers for multiples of ten (for example, thirty or ninety) are created by replacing the "y" with "ieth."

And we've got the one year celebration on the thirtieth of May.

Caption 30, FIFA U-20 World Cup New Zealand 2015 - New Zealand getting the word out

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This may seem like a lot to remember. However, for other numbers above twenty, only the last figure is written as an ordinal number:

English is the language of the twenty-first century. 

Caption 8, Strothoff International School - Imagefilm

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They have a wonderful internet café on the forty-seventh... forty-eighth floor.

Caption 67, An Apartment - in Japan

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Further Learning
Memorize the ordinal numbers from one to twenty with the help of this extensive list of ordinal numbers and find more examples on Yabla English.