日付や物の順番を英語で言う時には、 １は first ２は second といった言い方で表しますが、こういった言い方の数字のことを『 Ordinal numbers 』と言います。日本語（英文法）では序数と言われています。
物の数やお金、そして年齢を言う時には one やtwo といった英語を使います。これは Cardinal numbers と言われています。日本語では基数。
Ordinal numbersと発音の音声ファイル
1〜31までの ordinal numbers と、40〜90は10上がる毎に、100から100万までは位が上がる毎に ordinal number を表記しました。音声ファイルの再生の際は音量にご注意ください。
| 省略形 | Ordinal number | 発音 |
| 1st | first | |
| 2nd | second | |
| 3rd | third | |
| 4th | fourth | |
| 5th | fifth | |
| 6th | sixth | |
| 7th | seventh | |
| 8th | eighth | |
| 9th | ninth | |
| 10th | tenth | |
| 11th | eleventh | |
| 12th | twelfth | |
| 13th | thirteenth | |
| 14th | fourteenth | |
| 15th | fifteenth | |
| 16th | sixteenth | |
| 17th | seventeenth | |
| 18th | eighteenth | |
| 19th | nineteenth | |
| 20th | twentieth | |
| 21st | twenty-first | |
| 22nd | twenty-second | |
| 23rd | twenty-third | |
| 24th | twenty-fourth | |
| 25th | twenty-fifth | |
| 26th | twenty-sixth | |
| 27th | twenty-seventh | |
| 28th | twenty-eighth | |
| 29th | twenty-ninth | |
| 30th | thirtieth | |
| 31st | thirty-first | |
| 40th | fortieth | |
| 50th | fiftieth | |
| 60th | sixtieth | |
| 70th | seventieth | |
| 80th | eightieth | |
| 90th | ninetieth | |
| 100th | one hundredth | |
| 1,000th | one thousandth | |
| 10,000th | ten thousandth | |
| 100,000th | one hundred thousandth | |
| 1,000,000th | one millionth |
Ordinal Number の書き方
Ordinal number はアメリカ式とイギリス式で書き方が少し違います。当サイトではアメリカ式での書き方について説明します。
101からの ordinal number
101からは下2桁以下を ordinal number で書きます。例えば157という数字だと、one hundred fifty-seventh と書きます。
st / nd / rd / th などは最後の数字だけに付けて途中の数字には付けません。尚、ハイフンは必ず付けます。
例えば….
101st – one hundred first
204th – two hundred fourth
909th – nine hundred ninth
123rd – one hundred twenty-third
584th – five hundred eighty-fourth
101st は one hundred and first と書きます。584th だと five hundred and eighty-fourth と書きます。and を付けるんですね。
1,001からの書き方
1,001からの書き方ですが、数字を書くとき桁を表すのにコンマ( , )を書きますよね。それと一緒で ordinal number にもコンマを付けます。
例えば….
1,001st – one thousand, first
1,056th – one thousand, fifty-sixth
1,234th – one thousand, two hundred thirty-fourth
6,083rd – six thousand, eighty-third
9,765th – nine thousand, seven hundred sixty-fifth
10,001st – ten thousand, first
10,045th – ten thousand, forty-fifth
19,889th – nineteen thousand, eight hundred eighty-ninth
45,983rd – forty-five thousand, nine hundred eighty-third
199,999th – one hundred ninety-nine thousand, nine hundred ninety-ninth
1,999,999th – one million, nine hundred ninety-nine thousand, nine hundred ninety-ninth
19,999,999th – nineteen million, nine hundred ninety-nine thousand, nine hundred ninety-ninth
199,999,999th – one hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-ninth
1,999,999,999th – one billion, nine hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-ninth
1,999,999,999,999th – one trillion, nine hundred ninety-nine billion, nine hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-ninth
省略形の書き方
Ordinal number を書く時、英語のスペルを全部書くのも大変なので省略したのが、1st, 2nd, 3th, といった書き方です。数字のあとに ordinal number の最後の2文字（st / nd / rd /th）を付けて書きます。
first = 1st
second = 2nd
third = 3rd
fourth = 4th
eleventh = 11th
twelfth = 12th
twenty-ninth = 29th
one hundred twelfth = 112th
one hundred fifty – third = 153rd
one thousand eleventh = 1,011th
one thousand three hundred twelfth = 1,312th
one hundred thousandth = 100,000th