1 One；2 Two；3Three；4Four；5Five；6Six；7Seven；8Eight；9Nine；10Ten；11Eleven；12Twelve；13Thirteen；14Fourteen；15Fifteen；16Sixteen；17Seventeen；18Eighteen；19Nineteen；20Twenty。
应该是1—20的序数词的缩写形式，
1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 15th 16th 17th 18th 19th 20th 。
序数词都表示第几，后跟名词的单数形式。
例如，
my birthday is on January 20th.
this is my thirtieth birthday.
