What is a "open letter"

公开信,An open letter to President Habibie 致哈比比总统的公开信 an open protest; an open letter to the editor。公开抗议；给编辑的公开信。open account letter of credit 开出计算信用证 The importer has to apply to his bank to open a Letter of Credit。进口商须向本方的往来银行申请开具信用证。I started to open the letter,hoping to find some clue。我打开信,希望找到一些线索。