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letter (e.g. of complaint); letter to the editor; letter from a reader
contribution (to a newspaper, magazine, etc.)
新聞に投書を送ったことが何度かある。
I have sent a letter to the newspaper.
読者からの投書が毎日届いている。
Letters from readers arrive every day.
彼の投書が新聞に掲載されたそうだ。
I heard his letter was published in the newspaper.