平信

例句：

1. 1. We need to send a regular letter to our friend in Beijing.
   我们需要寄一封普通的信给在北京的朋友。
2. 2. She received an ordinary letter from her sister last week.
   她上周收到了姐姐的一封普通信件。
3. 3. The postman delivered the mail and handed me an ordinary letter.
   邮差送来了邮件，递给我一封普通的信。